LIBRARY OF CONGRESS. 



®]^p. - @api|ngl^ !f o, 

Shelf ..".."K-.L 

UNITED STATES OF AMERICA. 



COMPRESSED AIR. 



PRACTICAL INFORMATION UPON 



AIR-COMPRESSION AND THE TRANSMISSION AND 
APPLICATION OF COMPRESSED AIR. 



BY 



FRANK RICHARDS, Mem. A.S.M.E. 



FIRST EDITION. 
FIRST THOUSAND. 



NEW YORK : 

JOHN WILEY & SONS. 

London: CHAPMAN & HALL, Limited. 

1895. 




CiAO^ 



\ 



Copyright, 1895, 

BY 

FRANK RICHARDS. 



ROBERT DRUMMOND, ELECTROTYFER AND I'RINTER. NEW YORkC. 



6- :^^''^^ 



PREFACE. 



It is only proper to say that much of the matter con- 
tained in the following pages has appeared at intervals dur- 
ing the past two years in the columns of the American 
Machinist, The publication of the articles referred to and 
the remarks which they have elicited have served to em- 
phasize to me the too evident fact of the general scarcity 
of practical information about air-compression and the 
uses of compressed air, and the wide diffusion of misinfor- 
mation and prejudice upon this subject. In spite of it all 
the use of compressed air is rapidly spreading, and every- 
where with satisfaction to the users. I would gladly do 
what I can to extend the field of its usefulness, and I have 
so much faith in its powers that I believe that the best of 
all ways to advertise it is simply to tell the straight truth 
about it, and that I have tried to do. 

Frank Richards. 

New York, May, 1895. 



Ill 



COMPRESSED AIR. 



CHAPTER I. 

MECHANICAL VERSUS COMMERCIAL ECONOMY. 

Before considering the conditions under which air may 
be most economically compressed, having regard to the 
power cost alone, and the conditions relating to the trans- 
mission and application of the air, so that the most power 
may be realized, it seems proper to say something of the 
many applications of compressed air where the question of 
the actual power cost of the air, or of the actual amount 
of power realized from the air, seems to have little to do 
with the case. This is like asking a suspension of judg- 
ment, or a reservation of final decision upon the claims of 
compressed air, until the whole case has been presented ; 
and it seems to be rather necessary, because so many have ^ 
fallen into the habit of thinking only of the losses of power 
in the use of compressed air, and of arguing that because 
certain losses are proven, that therefore the employment of 
compressed air is not to be considered for any purpose. 
There are many men even in these days, and many intelli- 
gent engineers among them, who, to their own loss, will not 



2 COMPRESSED AIR. 

consider the claims of compressed air as a means of power 
transmission, because their minds have been so filled with 
this idea that its use entails enormous losses. That con- 
sideration settles it for them, and that is the libel from 
which compressed air suffers, so that it does not get a fair 
chance even to show what it can do. It is only another 
case of giving a dog a bad name ; and in this case it is 
a very good dog with a very bad name. It is the worst 
kind of a case to set right, and often the dog dies before 
justice is secured. In this case, however, there is not the 
slightest possibility of killing the dog or of shutting him out 
of sight, and the public cannot fail in the end to get hold of 
the facts as they are. 

The attitude of compressed air before the mechanical 
public, and especially the American mechanical public, has 
been a peculiar one all the way through. It has had no 
disinterested, all-around friends to look after its interests, 
nor interested ones either. There have been no men, and 
still less has there been any company of men, who have made 
the application of compressed air their business and have 
looked after it. Where is the General Compressed-Air Com- 
pany, in fact as well as in name, performing for compressed 
air such functions as more than one company are performing 
for electricity, and why is there not such a company ? Of 
the builders of air-compressors not one of them has been, 
not one of them is to this day, responsible for the econom- 
ical application of compressed air after the compression^ or 
apparently cares anything about it. This is not really to be 
wondered at, since the increase in the use of compressed air 
and in the demand for compressors has been so great and 
rapid, at least in the United States, that the compressor 
builders have been fully occupied in satisfying the demand. 



MECHANICAL VERSUS COMMERCIAL ECONOMY. 3 

Where compressed air has been used in this country, and 
where any thought has been given to economy in its use, 
the air-compressor, it would seem, has been ahiiost exclu- 
sively studied and talked about. In the progress of the 
steam-engine it has sometimes seemed that the steam-boiler 
has hardly received its proper share of attention. In the 
existing writings upon steam-engine economy it is probably 
safe to say that the engine engrosses ten times as much of 
the matter as the boiler does. In the case of compressed 
air the boiler, or compressor, gets ten times as much study 
and discussion as the engine or motor or other apparatus 
which uses the air. There are such vagaries of injustice in 
civilized communities. 

The impression which has got abroad of the waste of 
power in the use of compressed air has, curiously enough, 
been fostered and disseminated by the air-compressor peo- 
ple more than by any one else. We may say this with per- 
fect safety, for it strikes so generally that it hits no one in 
particular. The compressed air literature accessible to the 
general public consists principally of air-compressor cata- 
logues. The argument of the average catalogue runs like 
this : ** If you are going to use compressed air for any pur- 
pose, look out for the enormous losses of power to be en- 
countered, and which you are sure to experience, if you 
don't buy our compressor y And the result has been that 
many have " caught on '* to the terrible tale of the waste of 
power, and have helped to spread it far and wide. The 
argument is, of course, not maliciously meant ; but it has 
done more work, and somewhat different work, than that 
which was intended. 

But whether the employment of compressed air be eco- 
nomical or not, as far as the application of the power is 



4 COMPRESSED AIR. 

concerned, we do not propose to investigate that subject 
just here. We wish rather to call attention to the many- 
applications of compressed air where this question of power 
economy certainly does not apply. We may say, indeed, 
that in a large majority of the cases in which compressed 
air is used the question of power economy does not apply. 
Even in the use of compressed air for driving rock-drills in 
mines and tunnels, the field which still probably employs 
one half of all the compressed air that is used for mechani- 
cal purposes, the question of economy, or the comparative 
cost of operation, so far as it might determine the employ- 
ment or the rejection of the system, is not worth consider- 
ing, because in this field compressed air has no competitor, 
unless hand-power may be said to be one. In the use of 
compressed air for operating the brakes upon railway trains, 
a service which employs a greater number of air-compres- 
sors than any other line of service in the world, economy 
of power is not to be considered. Although it is notorious 
that the compressors employed for this service use five 
times the steam they should use for the work done, and 
ten times as much steam as the best-designed air-com- 
pressors of the present day would use for the same work, 
there would be no thought of throwing the air-brake off the 
trains if those " compressing-pumps," as they are familiarly 
called, used double or four times as much steam to do their 
work as they use now. Any one of several collateral con- 
siderations may easily take precedence of or assume greater 
weight and importance than the question of power economy 
in determining the employment of compressed air. The 
cases are few in which it is employed merely as a means of 
power transmission, and in competition with other means 
of power transmission, meeting them upon equal terms, and 



MECHANICAL VERSUS COMMERCIAL ECONOMY, 5 

with no other consideration but that of the comparative 
economy with which the power is transmitted. There are 
many cases that are clearly cases of power transmission, or 
cases of work to be done at a distance from the source of 
power, where the question as to whether the power is to be 
applied continuously or intermittently may be a most im- 
portant factor in the general problem. Compressed air is 
unique among all power transmitters — at least among long- 
distance power transmitters — in that it is always and in- 
stantly ready to do its work to its full capacity, and yet 
that it charges nothing for its services except when it is 
actually employed. Other transmitters may or may not 
propose to do the actual work a little cheaper, but they 
expect to be an expense to their employer for maintenance 
when not employed. In the pneumatic switch and signal 
service a single air-compressing plant is capable of operating 
the switches and signals upon a section of railway twenty 
miles long ; and when the pipes are once filled with air at 
the required pressure, that air does not condense or suffer 
loss or deteriorate in any way except as it is used. Elec- 
tricity makes open confession of its inability to do this 
intermittent work, in that while in this switch and signal 
service it is actually employed to give the wink to the air 
as to w^hat is wanted, the air has to do the work. It need 
not be said that steam could not do this work, for its 
strength w^ould all be turned to water before the end of the 
pipe was reached. 

How little weight the actual power economy may have 
in determining the employment of compressed air for a 
given purpose is suggested by the conditions of the pneu- 
matic postal service. In the cities of Europe pneumatic 
postal transmission has been an established commercial sue- 



6 COMPRESSED AIR. 

cess for years, and recently the same most gratifying experi- 
ence is realized at the Philadelphia post-office. The appli- 
cation of the air is not mechanically economical, only ten 
per cent of the power employed being applied to the work 
of moving the carriers, while ninety per cent is " wasted " 
in accelerating the air and in its friction. 

The " compressing-pump " of the air-brake service being 
now familiar to all railroad men, and being at hand or eas- 
ily procurable by all railroad shops, has been of late years 
always ready in those shops with its supply of compressed 
air, and this ready supply of compressed air has led to the 
general employment of it in railroad shops and in railroad 
service for a variety of uses. Whatever it is tried for, its 
use for that purpose continues, and one thing leads to an- 
other. These uses of compressed air in railroad shops con- 
tinually increase, not because the air is more adapted to 
railroad use than to any other, but because the air is there. 
Where the air is once used in one of these shops its use in- 
creases, the volume of air used increases, and the increas- 
ing demand for the air is met by setting up additional 
" compressing-pumps " in succession, until in some in- 
stances as many as eight of them have been employed to- 
gether to supply a single shop. It is evident here that 
power economy could have little to do with the case, or 
some thought would be given to the economical compres- 
sion of the air, and a good air-compressor would take the 
place of the air-brake pumps as a means of supply. This 
substitution is now in progress, we are happy to say, in 
many shops with most gratifying results. 

A recent floating paragraph tells how the master me- 
chanic of one of the railroads does his whitewashing by 
compressed air : " An old freight-car has been fitted with 



MECHANICAL VERSUS COMMERCIAL ECONOMY. 7 

three air-brake pumps and two reservoirs," instead of one 
air-compressor and one reservoir or receiver, " the pumps 
being driven by steam from an engine (locomotive) and a 
pressure of 40 pounds being maintained in the reservoirs. 
The car and engine are run upon the track alongside the 
building to be whitewashed. By a system of piping the air 
is carried into the building, and to the long iron nozzles 
used by the men in applying the fluid. Each man has an 
iron tube with a funnel-shaped end, from which the white- 
wash is sprayed upon the woodwork. To each nozzle are 
attached two lines of hose, one supplying the air and the 
other the whitewash. The air rushes into the cylinder of 
the nozzle, and its pressure causes a suction that brings up 
a stream of whitewash and at the same time expels it in 
the form of spray.'* There can be no doubt of the success 
of this scheme ; but if it pays to use a locomotive, a freight- 
car, three brake-pumps, two reservoirs, and all the rest of it, 
for such a job, compressed air surely must be cheap at any 
price, or there must be economy in compressed air if eco- 
nomically compressed and wisely applied. 

As to how much the question of power economy may 
have to do with the remunerative use of compressed air is 
suggested by an article in Machinery, September, 1894, 
describing the various uses to which compressed air is put 
in the West Shore R. R. shop at Frankfort, N. Y. " It is 
stated that the entire power cost for running the air-com- 
pressors to supply the whole shop is not more than ten 
cents per day, while the actual saving in labor effected is 
from fifteen to twenty dollars per day. In the article re- 
ferred to, among the many applications of compressed air 
in the above shop mention is made of a machine for (by 
the aid of compressed air) putting the couplings into the 



8 COMPRESSED AIR, 

ends of air-brake hose. ^* This little machine has a record 
of putting together one hundred in one hour and five min- 
utes, against twenty-five in one day by hand, and actually 
paid for its entire cost in one day's application." 

Simply give compressed air a chance and it will quickly 
demonstrate its value. The progress in the use of com- 
pressed air thus far seems plainly to indicate that its wider 
application is promoted more by having a supply of it ready 
at hand, or easy to get, than by showing how cheaply it can 
be furnished. Those who are introducing small, cheap 
compressors that work with reasonable economy are doing 
excellent missionary work for compressed air. Farther 
along, as large, permanent compressing-plants are estab- 
lished, we may believe that the minute economies will re- 
ceive the consideration which they deserve. However the 
air may be used, and however profitable it may be to use it, 
it will always be in order to get it as cheaply as possible, 
and economy in air-compression must always be a clear 
gain. 



CHAPTER 11. 
DEFINITIONS AND GENERAL INFORMATION. 

At the beginning it would seem to be well to get to- 
gether for use or reference the general facts in relation to 
air and to the phenomena attending its compression. As 
this work is intended for the greatest good of the greatest 
number, being for popular use, or for the use of those who 
will practically have to do with compressed air, and as it is 
in no sense for the use of expert scientists, the common 
standards of weight and measurement will be employed 
wherever it is possible to use them intelligibly. For tem- 
peratures the Fahrenheit scale will be used exclusively. 
All measures of length or distance will be given in feet 
and inches, and weights in pounds avoirdupois. Where 
pressures are referred to, they will be the pressures as indi- 
cated upon a common pressure gauge, or the pressures 
above that of the atmosphere. The absolute pressure, of 
course, is the gauge pressure plus the pressure of the at- 
mosphere at the given time and place, this atmospheric 
pressure being usually taken as 14.7 lbs. at the sea-level. 
It will be necessary to refer to absolute pressures occasion- 
ally, but we trust that no misunderstanding will occur. 

Air is composed of 23 parts by weight of oxygen and 77 
parts by weight of nitrogen. By volume the proportions 
are 21 parts of oxygen and 79 parts of nitrogen. Although 

9 



10 COMPRESSED AIR. 

oxygen is thus less than one quarter of the air, it is much 
more studied and written about and is apparently of much 
more use and importance than the larger constituent. It 
may be that the functions of nitrogen are not yet very fully 
or clearly understood. It certainly has not been considered 
deserving the study, nor has it received the attention that 
oxygen has received. Oxygen is the active partner in the 
combination. A friend who is blessed with more knowl- 
edge in this line than is possessed by the writer suggests 
that oxygen is made for the mechanic and nitrogen for the 
farmer. 

We shall frequently use the term " free air '* as we go 
along. The term free air in contradistinction from com- 
pressed air is only used as a matter of convenience and 
custom. Free air, or air at atmospheric pressure, is really 
compressed air, or air subjected to pressure, as truly as air 
at loo lbs. pressure is compressed air, and its volume, press- 
ure, and temperature are subject to the same laws. By free 
air, as the term is commonly used, is meant air at atmos- 
pheric pressure and at ordinary temperature, and it is the 
air as we receive it when we begin the operation of air. 
compression. It is free air, or it should be free air, when 
first admitted to the air-compressing cylinder, and it is not 
free air again until it is exhausted or discharged into the 
atmosphere, when it has done its work and we have no 
further use for it or connection with it. 

The condition in which our free air is received is not by 
this general term accurately defined in any of its particu- 
lars, either as to pressure, volume, or temperature. The 
pressure and volume of the air may vary with the altitude 
or location, or with the barometric reading in any given 
location, or, again, the volume may vary with the tempera- 



DEFINITIONS AND GENERAL INFORMATION, II 

ture. The temperature may vary with the changes of the 
seasons or with the special surroundings. For general 
purposes we shall assume our free air to be at the usual sea- 
level atmospheric pressure of 14.7 lbs., absolute, and at a 
temperature of 60°. 

We shall have to refer constantly to temperatures, and, 
as said above, the Fahrenheit scale will be used exclusively, 
and usually the temperatures will be the sensible tempera- 
tures, or those indicated by the common Fahrenheit ther- 
mometer, 32° being the melting-point of ice, or the point 
where water changes from the solid to the liquid state, and 
212° being the point where it changes from the liquid to 
the gaseous state. The boiling-point is in practice quite a 
variable one, and depends entirely upon the pressure sur- 
rounding the water> 212° being the boiling-point only at 
ordinary atmospheric pressures near the sea-level. Water 
may theoretically be made to boil at any temperature above 
the freezing-point by sufficiently reducing the atmospheric 
pressure to which it is exposed. The range of the Fahren- 
heit scale between the melting-point of ice and the boiling- 
point of water is 180 degrees. 

We shall have frequent occasion to refer to absolute tem- 
peratures. Absolute temperature by the Fahrenheit scale is 
the temperature as indicated by the thermometer plus 461 
degrees. Thus at 60° by the thermometer the absolute 
temperature is 60 -]- 461 — 521. At zero the absolute 
temperature is o + 461 = 461. At temperatures below zero 
the absolute temperatures are also determined in the 
same way, by simple addition. Thus, if the temperature 
by the thermometer is 30" below zero, or •— 30°, the ab- 
solute temperature will be — 30 + 461 — 431. In all 
questions relating to the volume, pressure, or weight of 



12 COMPRESSED AIR, 

air, whether compressed or not, the absolute temperature 
of the air has an important bearing, as the volume of the 
air will vary directly as the absolute temperature, and the 
pressure and the weight of the air will all have changing 
relations. If the absolute temperature of the air is in- 
creased, the volume will be increased in the same pro- 
portion, the pressure remaining unchanged. So if the 
absolute temperature of the air be diminished, the vol- 
ume will be diminished in the same way. The relations 
of volume, pressure, and temperature of air are thus sum- 
marized : 

1. The absolute pressure of air varies inversely as the 
volume when the temperature is constant. 

2. The absolute pressure varies directly as the absolute 
temperature when the volume is constant. 

3. The volume varies as the absolute temperature when 
the pressure is constant. 

4. The product of the absolute pressure and the volume 
is proportional to the absolute temperature. 

A cubic foot of dry air at atmospheric pressure and at 
any absolute temperature will weigh 39.819 lbs. divided by 
the absolute temperature. Thus at 60° a cubic foot of air 
weighs 39.819 -^ (60 + 461) = .0764 lb. So, inversely, the 
volume of i lb. of air at atmospheric pressure and at any 
absolute temperature may be ascertained by dividing the 
temperature by 39.819. 

Thus at 60° as before 521 -^ 39.819 ~ 13.084 cu. ft. 

The following table (I), from Appleton's Applied Me- 
chanics, shows the weight and volume of air at different 
temperatures. 

If the temperature and the pressure of air both vary the 
constant 2.7093 multiplied by the absolute pressure in lbs. 



DEFINITIONS AND GENERAL INFORMATION, 1 3 

Table I. 

TABLE OF THE WEIGHT AND VOLUME OF DRY AIR AT ATMOSPHERIC 
PRESSURE AND AT VARIOUS TEMPERATURES. 

From Appleton's Applied Mechanics. 



Temperature, 


Weight of 
One Cubic Foot 


Volume of 
One Pound in 


Degrees Fahr. 


in Pounds. 


Cubic Feet. 


O 


.0863 


11.582 


ID 


.0845 


11.834 


20 


.0827 


12.085 


30 


.0811 


12.336 


32 


.0807 


12.386 


40 


.0794 


12.587 


50 


.0779 


12.838 


60 


.0764 


13.089 


70 


.0750 


13.340 


80 


.0736 


13.592 


90 


.0722 


13.843 


100 


.0710 


14.094 


IIO 


.0697 


14.345 


120 


.0685 


14.596 


130 


.0674 


14.847 


140 


.0662 


15.098 


150 


.0651 


15.350 


160 


.0641 


15.601 


170 


.0631 


15.852 


180 


.0621 


16.103 


190 


.0612 


16.354 


200 


.0602 


16.605 


210 


.0593 


16.856 


212 


.0591 


16.907 



per sq. in. and divided by the absolute temperature will 
give the weight of a cubic foot. 

What will be the weight of i cu. ft. of air at 60 lbs. 
pressure and 100° tenaperature ? 

2.7093 X (60 + 14.7) -4- (100 + 461) = .3607 lb. 

The volume of i pound of air may be obtained by di- 
viding the absolute temperature by the absolute pressure 
and dividing this by the same constant, 2.7093. 



14 COMPRESSED AIR, 

What will be the volume of i lb. of air at 75° tempera- 
ture and 50 lbs. pressure ? 

(75 + 461) -^ (50 + 14.7) -^ 2.7093 =r 3.052 lbs. 

If the temperature of the air is changed from one ab- 
solute temperature T to another absolute temperature /, 
the volume remaining constant, the resulting absolute 
pressure p may be obtained from the original absolute 
pressure P by the simple proportion T \ t w P \ p^ or 

It is not supposed that heat is an actual existence any 
more than sound or light is ; still it is very necessary, 
especially in all matters relating to compressed air, to be 
able to accurately measure and state the effects of heat, 
and to have some unit or standard of measurement and 
comparison. The unit of heat generally adopted is that 
quantity of heat that will raise the temperature of one 
pound of water one degree. One unit of heat if applied to 
one pound of anything else will not have precisely the same 
effect in raising the temperature that it has when applied 
to water. More heat is required to raise the temperature 
of one pound of water one degree than is required for any 
other substance. The heating effect of a unit of heat 
applied to different substances is found to vary widely, and 
the special quantity of heat required to raise the tempera- 
ture of one pound of any substance one degree is known as 
its specific heat. The specific heat of water being i, the 
specific heat of air is .2377, or the same unit of heat that 
would raise the temperature of one pound of water one 
degree would raise the temperature of one pound of air 
more than four degrees. The application of heat to air or 
to any elastic fluid may have either of two effects. It may 



DEFINITIONS AND GENERAL INFORMATION, 1 5 

increase the volume while the pressure remains constant, or 
it may increase the pressure while the volume remains con- 
stant. The specific heat will be quite different in the two 
cases. The specific heat of air — .2377, as given above — is 
its specific heat at constant pressure, and the heat applied 
in this case exhibits its effect in increasing the volume of 
the air. If the air be confined so that there can be no in- 
crease of volume, its specific heat is only .1688, or about 
one sixth that of water. Heat applied to air at constant 
volume increases the pressure of the air. If heat be applied 
to air under constant pressure, raising its temperature from 
32° to 212°, the increase in volume will be from i to 1.367 ; 
and if heat be applied to air at constant volume, raising its 
temperature, as before, from 32° to 212°, the increase in 
absolute pressure will be from i to 1.365, the numerical 
result being practically alike in the two cases, but the heat 
expended will be as .2377 : .1688, or nearly one half more 
in one case than in the other. 

When air is compressed, or when its volume is reduced 
by the application of force, the temperature of the air is 
raised. This phenomenon occurs entirely regardless of the 
time employed in the compression. If during the compres- 
sion the air neither loses nor gains any heat by communi- 
cation with any other body, the heat generated by the act 
of compression remaining in the air and increasing its 
temperature, then the air is said to be compressed adia- 
batically, and such compression is adiabatic compression. 
When the pressure is removed from the air and it is allowed 
to expand, its temperature falls, and if the air during this 
operation receives no heat from without, it is said to expand 
adiabatically. Adiabatic compression or expansion of air 
is compression or expansion without loss or gain of heat by 



1 6 COMPRESSED AIR, 

the air. This expression " without loss or gain of heat," it 
will be noticed, does not mean maintaining a constant tem- 
perature, but precisely the reverse of that. 

If during compression the air could be kept at a con- 
stant temperature by the abstraction of the heat as fast as 
it was generated, the air would then be said to be com- 
pressed isothermally. In isothermal compression or ex- 
pansion the air remains at a constant temperature through- 
out the operation. 

The rate of increase in the temperature of air during 
compression is never uniform. The temperature rises faster 
during the earlier stages of the compression than when the 
higher pressures are reached. Thus in compressing from 
I atmosphere to 2 atmospheres the increase of temperature 
will be greater than in compressing from 2 to 3 atmospheres, 
and so on. The rate of increase of temperature also varies 
with the initial temperature. The higher the initial tem- 
perature the greater will be the rate of increase at any point 
and throughout the compression. 

Attention is called to the diagram which appears as a 
frontispiece to this work. It is taken from " Compressed- 
Air Production," by W. L. Saunders, C.E. The writer 
hereof is in the habit of keeping this diagram in sight, and 
finds it suggestive and handy in the off-hand solution of 
many questions that arise. It would seem to be worthy of 
a rather fuller explanation than Mr. Saunders has favored 
us with. The diagram in fact comprises two distinct dia- 
grams, the one showing the temperature of the air, and the 
other showing the volume of the air at different stages of 
compression. If the diagrams are understood, there is no 
danger of confusing the one with the other, and as many of 
the lines do service for both diagrams, we are able to get 



DEFINITIONS AND GENERAL INFORMATION. T/ 

much from a small space. Compression is supposed to 
commence at the left of the diagram with any given volume 
of air at atmospheric pressure. As the compression pro- 
ceeds the successive stages of pressure are indicated by the 
series of vertical lines. Beginning at the extreme left witn 
the gauge pressure at zero, o, as shown at the bottom of 
the diagram, or with an absolute pressure of i atmosphere, 
as indicated at the top of the diagram, when the first 
vertical line is reached the air is then compressed to 2 
atmospheres, as shown by the figure at the top, or to 14.7 
lbs. gauge pressure, as shown by the figures at the bottom. 
When the next vertical line is reached, the air has been 
compressed to 3 atmospheres, absolute, or to 29.4 lbs. 
gauge pressure, and so on, the diagram extending to 21 
atmospheres, or 294 lbs. gauge pressure at the extreme 
right. 

In connection with the compression of air the important 
facts to be known are the temperature of the air when any 
given pressure is reached, and also the relative volume of 
the air when compressed to any given pressure, and these 
points it is the function of the diagram to show. The 
curved lines running from the lower left-hand corner A of 
the diagram are the lines of temperature, and they indicate 
by their height above the base-line AB the temperature of 
the air at any stage of the compression. It is assumed in 
the use of this part of the diagram that the air is com- 
pressed adiabatically, or that it loses none of the heat of 
compression during the operation. The several horizontal 
lines of the diagram serve to indicate by their height above 
the base-line AB the temperature attained. The space 
between any two adjacent horizontal lines represents 100° 
of temperature. Thus the temperature at the base-line 



1 8 COMPRESSED AIR, 

AB being zero the temperature at the first line above 
it is ioo°, and so on. The temperatures indicated by the 
lines are shown by the figures at the left of the diagram 
along the vertical starting-line AC. Intermediate tempera- 
tures falling between the lines may be estimated by the 
relative distances from the lines. The temperature of the 
air at any stage of the compression depends upon its initial 
temperature. The higher the initial temperature is the 
higher will be the temperature throughout the compression. 
The diagram gives temperature-lines for the compression 
of air from the several initial temperatures of o°, 60°, and 
100°. These lines show, as noted above, that the higher 
the initial temperature the more rapid is the rise through- 
out the compression. Thus comparing the compression- 
line from 0° with the line of compression from 100° we 
notice that when the air has been compressed from i 
atmosphere to 10 atmospheres the original difference of 
100 degrees has become 200 degrees, and when the com- 
pression is carried to 20 atmospheres, the difference has 
become 250 degrees. 

The curved lines starting downward from the point C at 
the upper left-hand corner of the diagram represent the 
volume of any unit of air after compression to any given 
pressure. The upper curved line represents the resulting 
volume after compression without any cooling of the air 
during compression, or with all the heat of compression 
remaining in the air. This is the adiabatic compression- 
line. The lower or inner of the two curved lines repre- 
sents the volume of air after compression to any given 
pressure, and with all the heat of compression abstracted 
immediately as it is developed, or with the air constant at 
the initial temperature throughout the compression. This 



DEFINITIONS AND GENERAL INFORMATION, 1 9 

is the line of isothermal compression. The initial tem- 
perature of the air whether it is compressed isothermally or 
adiabatically is not a factor in determining the resulting 
volume. The total height of the starting-line ^C represent- 
ing I volume of air, the volume at any stage of the com- 
pression is represented by the vertical height of either 
curved line at that point. The several horizontal lines of 
the diagram serve to indicate the height, and by the height 
the volume, of the air at any pressure, and in this relation 
the lines have an entirely different function from that 
borne by them in relation to the lines of temperature. 
The initial volume is assumed to be divided into ten equal 
parts, and the space between any two adjacent horizontal 
lines represents one tenth of the original volume. Thus 
the first horizontal line below the top line CD represents 
nine tenths of the initial volume, the next line below in- 
dicates eight tenths of the original volume, and so on. 
These values are indicated by figures at the right of the 
diagram along the line DB. 



20 



COMPRESSED AIR, 
Table II. 



TABLE OF VOLUMES, MEAN PRESSURES, TEMPERATURES, ETC., IN THE 
OPERATION OF AIR-COMPRESSION FROM I ATMOSPHERE AND 
60° FAHR. 



I 


2 


3 


4 


5 


6 


7 


8 


9 


10 


II 






. 


4-1 ' 


^ 


b/D • L 


hr • 


. 




t 

I 


i 

tn 

(A 


«3 

6 

< 






c 

< 
•a 

0) <u 


art 

c 
<u 

O) >- v- 

!S<2 
£ 2 


K 

<u 


-c 

lilt 


a; - <u 


en 

(U 

88 

4J(J 



C 


en 

1 


tie 


3 
J2 


to. 




So 

3 






ilA 


3 
rt 





<: 


£ 


> 


> 

I 








% 


^ 


fe 








14.7 


I 


I 








60 





I 


15-7 


1.068 


.9363 


-95 


.96 


.975 


.43 


.44 


7^ 


I 


2 


16.7 


1. 136 


.8803 


.91 


1.87 


1.91 


.95 


.96 


80.4 


2 


3 


17.7 


1 .204 


.8305 


.876 


2.72 


2.8 


1.4 


1. 41 


88.9 


3 


4 


18.7 


1.272 


.7861 


.84 


3-53 


3-67 


1.84 


1.86 


98 


4 


5 


19.7 


1-34 


.7462 


.8i 


4-3 


4-5 


2.22 


2.26 


106 


5 


10 


24.7 


1.68 


-5952 


.69 


7.62 


8.27 


4.14 


4.26 


145 


10 


15 


29.7 


2.02 


• 495 


.606 


10.33 


11.51 


5-77 


5.99 


178 


15 


20 


34-7 


2.36 


.4237 


-543 


12.62 


14.4 


7.2 


7.58 


207 


20 


25 


39-7 


2.7 


.3703 


•494 


14.59 


17.01 


8.49 


9 05 


234 


25 


30 


44.7 


3-04 


.3289 


.4638 


16.34 


19.4 


9.66 


10.39 


255 


30 


35 


49-7 


3.381 


•2957 


.42 


17.92 


21.6 


10.72 


"-59 


281 


35 


40 


54-7 


3.721 


.2687 


.393 


19.32 


23.66 


II. 7 


12.8 


302 


40 


45 


59-7 


4.061 


.2462 


.37 


20.52 


25.59 


12.62 


13 95 


321 


45 


50 


64.7 


4 401 


.2272 


■35 


21.79 


27-39 


13.48 


15 .05 


339 


50 


55 


69.7 


4.741 


.2109 


•331 


■21. TJ 


29.11 


14-3 


15.98 


357 


55 


60 


74 7 


5.081 


.1968 


.3144 


23.84 


30.75 


1505 


16.89 


375 


60 


65 


79-7 


5-423 


.1844 


.301 


24.77 


31.69 


15.76 


17.88 


389 


65 


70 


84.7 


5.762 


• 1735 


.288 


26 


33.73 


16.43 


18.74 


405 


70 


75 


89.7 


6.102 


.1639 


.276 


26.65 


35-23 


17.09 


19-54 


420 


75 


80 


94-7 


6.442 


• 1552 


.267 


27-33 


36.6 


17-7 


20.5 


432 


80 


85 


99-7 


6.782 


.1474 


.2566 


28.05 


37.94 


18.3 


21.22 


447 


85 


90 


104.7 


7.122 


.1404 


.248 


28.78 


39.18 


18.87 


22 


459 


90 


95 


109.7 


7.462 


.134 


.24 


29-53 


40.4 


19.4 


22.77 


472 


95 


100 


114. 7 


7.802 


.1281 


.232 


30.07 


41.6 


19.92 


23 -43 


485 


100 


105 


119. 7 


8.142 


.1228 


.2254 


30.81 


42.78 


20.43 


24.17 


496 


105 


no 


124.7 


8.483 


.1178 


.2189 


3^-39 


43-9^ 


20.9 


24.85 


507 


110 


"5 


129.7 


8.823 


• I 133 


.2129 


31.98 


44-98 


21.39 


25.54 


5^8 


"5 


120 


134-7 


9.163 


.1091 


.2073 


32-54 


46.04 


21.84 


26.2 


529 


120 


125 


^39-7 


9-503 


.1052 


.202 


33-07 


47.06 


22.26 


26.81 


540 


125 


130 


144.7 


9-843 


.1015 


.1969 


33-57 


48.1 


22.69 


27.42 


550 


130 


135 


149-7 


10. 183 


.0981 


•'922 


34-05 


49.1 


23.08 


28.05 


560 


135 


140 


154-7 


10.523 


•095 


.1878 


34.57 


50.02 


23.41 


28.66 


^r 


140 


145 


159-7 


10.864 


. 092 1 


.1837 


35-09 


51 


23.97 


29.26 


580 


145 


150 


164.7 


11.204 


.0892 


.1796 


35.48 


51.89 


24.28 


29.82 


589 


150 


160 


174-7 


11.88 


.0841 


.1722 


36.29 


53.65 


24.97 


30.91 


607 


160 


170 


184.7 


12.56 


.0796 


•1657 


37.2 


55-39 


25.71 


32.03 


624 


170 


180 


194.7 


13 24 


.0755 


.1595 


37-96 


57.01 


26.36 


33.04 


640 


180 


190 


204.7 


13.92 


.0718 


•154 


38.68 


58.57 


27.02 


34.06 


657 


190 


200 


214.7 


14.6 


.0685 


.149 


39 42 


60.14 


27.71 


35.02 


672 


200 



CHAPTER III. 
A TABLE FOR AIR-COMPRESSION COMPUTATIONS. 

The accompanying Table II, which the writer uses con- 
stantly in his own practice, will be found convenient for 
working up indicator diagrams from air-compressing cylin- 
ders, and in general computations relating to air-compres- 
sion. It should require little explanation. Throughout 
the table the air is assumed to be compressed from the 
normal pressure of i atmosphere, — 14.7 pounds, — and 
from an initial temperature of 60° Fahrenheit. The first 
three columns of the table are of course different forms of 
the same thing — the pressure to which the air is compressed. 
The last column of the table is also the same as the first 
merely for the convenience of following the lines of figures. 
The first column gives the pressures as they would actually 
be shown by a steam- or pressure-gauge. It would be the 
actual available working pressure of the air after compres- 
sion. The second column, or the absolute pressure, is ob- 
tained by adding the normal atmospheric pressure — 14.7 
pounds — to the gauge pressure. The third column, show- 
ing the pressure in atmospheres, is obtained by dividing 
the absolute pressure by the normal atmospheric pressure 
— 14.7 pounds. 

Column 4 gives the volume of air (the initial volume 
being i) after isothermal compression to the given press- 

21 



22 COMPRESSED AIR. 

ure ; that is, assuming that the temperature of the air has 
not been allowed to rise during the compression, or that, if 
the air has not been completely cooled during the com- 
pression, it has been cooled to the initial temperature 
after the compression. In this case the volume is assumed 
to be inversely as the absolute pressure, which is very 
nearly correct. The figures in column 4 are in fact re- 
ciprocals of those in column 3, and they are obtained by 
dividing i by the several successive values in column 3. 
Thus, for a gauge pressure of 50 pounds, the volume by 
isothermal compression should be i -^ 4.401 = .2272, as 
given in column 4. So far as the air-compressor is con- 
cerned, this column represents an eternally unattainable 
ideal. There is, and as far as we can see there can be, 
no absolutely isothermal compression. Some '* hydraulic " 
compressors are claimed to accomplish it, but while there 
is no promise of their practical success thy have no right 
to stand in evidence. The compressed volume while in 
the compressing cylinder, or at the moment of discharge, 
will always be greater than given in column 4 for the cor- 
responding pressure, because it is impossible to compress 
air- and at the same time abstract all the heat of com- 
pression from it. This column does, however, give the 
volume of air that will be realized if the air is trans- 
mitted to some distance from the compressor, or if it is 
allowed to give up its heat in any way before it is used. 
Air will be found to lose its heat very rapidly, and this 
column may be taken to represent the volume of air after 
compression actually available for the purpose for which 
the air may have been compressed. 

Column 5 of the table gives the volume of air at the 
completion of the compression, assuming that the air has 



AIR-COMPRESSION COMPUTATIONS. 23 

neither lost nor gained any heat during the compression, 
and that all the heat developed by the compression remains 
in the air. This column shows the air more nearly as 
the compressor usually has to deal with it, although the 
condition represented by this column — adiabatic compres- 
sion—is never actually realized, any more than isothermal 
compression is realized. In any compressor the air will 
jose some of its heat during the compression, and the air is 
never as hot during the compression, nor at the completion 
of the compression, as theory says that it should be. The 
theory is all right, but the air loses some of its heat. The 
slower the compressor runs the better chance the air has to 
give up some of its heat, and consequently the smaller will 
be its volume all through the operation, and the less will be 
the power required. High or excessive speeds are not in 
the interest of economy for many reasons. If the cylinder 
and the cylinder-heads are water-jacketed, the cooling of 
the air and the reduction of volume and of mean effective 
resistance will be quite appreciable ; but in general prac- 
tice the actual volumes of air at the completion of compres- 
sion will be found to be nearer the figures given in column 
5 than to those in column 4. 

Column 6 gives the mean effective resistance to be over- 
come by the air-cylinder piston in the stroke of compres- 
sion, assuming that the air throughout the operation re- 
mains constantly at its initial temperature — isothermal 
compression. Of course the air never will remain at con- 
stant temperature during compression, and this column 
remains the ideal to be kept in view and striven for and 
continually approximated in economical compression. 

Column 7 gives the mean effective resistance to be over- 



24 COMPRESSED AIR, 

come by the piston for the compression stroke, supposing 
that there is no cooling of the air during the compression — 
adiabatic compression. As we have seen, there is more or 
less — generally less, but always some — cooling of the air 
during its compression, so that the actual mean effective 
resistance will always be somewhat less than as given in 
this column ; but for computing the actual power required 
for operating air-compressor cylinders the figures in this 
column for the given terminal pressures may be taken and 
a certain percentage added for friction, — say 5 per cent, 
— and the result will represent very closely the power re- 
quired by the compressor. In proposing to add 5 per cent 
for friction we do not mean that the total friction of a 
steam-actuated air-compressor will be only 5 per cent, for 
it will probably be more than 10 per cent, but part of 
this TO per cent will have been compensated for by the 
partial cooling of the air during the compression. In 
some compressors now in use it is probable that so much 
cooling is effected during the compression, and so much 
power is saved thereby, as to entirely compensate for the 
friction of the machine, and nothing need be added to the 
result. The values given in columns 6 and 7 are of course 
used in computing the horse-power of an air-compressing 
cylinder precisely as the mean effective pressure per 
stroke in a steam-cylinder is used in computing its power. 
In the steam-cylinder the computation gives the power 
developed by the steam, and the same system of com- 
putation applied to the air-cylinder gives the power used in 
the compression. 

Having an air-compressing cylinder 20" dia. X 2' stroke 
at 75 revolutions per minute, or 300' piston speed, com- 



AIR-COMPRESSION COMPUTATIONS, 25 

pressing air adiabatically to 75 lbs., the horse-power used 
will be computed as follows : 

20' X .7854 X 35.23 X 300 -^ 33,000 = 100 H.-P. 

It may be proper to suggest here one caution as to the 
use of the mean effective pressures given in columns 6 and 
7. The pressures given being for compression to different 
pressures from an initial pressure of i atmosphere, it 
does not follow that those values will be correct for com- 
putations in compound compression, or for compression 
from any other initial pressure but that of i atmosphere. 
Thus in column 7 the M.E.P. for compressing from i 
atmosphere to 50 lbs. gauge pressure is 27.39. ^^ ^^^ ^^se 
the pressure of the air compressed is increased 50 lbs., but 
it does not follow that we can take air at 50 lbs. and com- 
press it to 100 lbs. with the same mean effective pressure. 
In the latter case the M.E.P. required would be 40.33, or 
47 per cent greater than in the former case. 

Column 8 gives the mean effective resistance for the 
compression part only of the stroke in compressing air 
isothermally from a pressure of i atmosphere to any 
given pressure. This at once calls our attention to the two 
distinct operations involved in practical air-compression : 
the actual compression of the air to the given pressure, and 
the' delivery or expulsion of the air from the cylinder after 
the full pressure is attained. These two operations corre- 
spond inversely to the two operations occurring in the 
cylinder of a steam-engine : the admission of the steam, 
where it is sustained at approximately full pressure until 
the point of cut off, and the expansion of the steam from 
the point of cut off to the termination of the stroke, the 
expansion period in the steam-cylinder of course corre- 



26 COMPRESSED AIR. 

spending inversely with the compression in the air-cylinder, 
and the admission of the steam corresponding with the 
delivery of the air. 

It will be noticed that the mean effective pressures in 
columns 8 and 9, for the compression part only of the 
stroke, are much lower than those in columns 6 and 7 for 
the whole stroke, but when to the work of the compression 
part of the stroke is added the work of delivery, the values 
will be found to correspond very nearly. Thus when com- 
pressing adiabatically to 50 pounds gauge pressure the 
volume of air delivered will be (column 5) .35 of the origi- 
nal volume, or .35 of the stroke for each cylinderful of 
free air, sot hat the pressure or resistance for .35 of the 
stroke will be 50 lbs., while for the compression part of the 
stroke — i — .35 = .65 — the resistance will be 15.05, as given 
in column 9. Then (15.05 X .65) + (50 X .35) = 27.28, 
which corresponds as well as could be expected with the 
value in column 7 for the whole stroke — 27.39. 

There is also to be observed a less proportional differ- 
ence between the values in columns 8 and 9 than between 
those in columns 6 and 7, but this also will be found to be 
compensated for by the differences in terminal volume for 
isothermal or for adiabatic compression and the different 
proportion of the stroke occupied by the full pressure of 
delivery. Thus comparing the figures for isothermal com- 
pression with those just given for adiabatic compression, 
compressing to 50 lbs. as before, we have : (13.48 X .7728) 
+ (50 X .2272) = 21.78, a result which may be said to be 
identical with the value 21.79 for the whole stroke, as given 
in column 6. 

Columns 8 and 9, as will be referred to later, will be 
found serviceable in computing the power used in the first 



AIR-COMPRESSION COMPUTATIONS, 2^ 

Stage of compound compression, where generally the entire 
function of the first cylinder is that of compression only, 
its total contents from the beginning to the end of the 
stroke being simply compressed into the volume contained 
in the smaller cylinder, and there being no part of the 
stroke properly occupied in delivery or expulsion at any 
completed pressure. 

Column lo gives the theoretical temperature of the air 
after compression, adiabatic, to the given pressure. As we 
have remarked elsewhere, the actually observed tempera- 
ture in these cases is never as high as the theoretical tem- 
perature. This is not that the theory is incorrect, for, as 
usual, the theory is more nearly correct than ** practical *' 
people are wont to allow. If the temperature of the com- 
pressed air by observation is not found to correspond with 
the figures as given, it is only because the air is being cooled 
by conduction or radiation even while it is being heated by 
compression. 



CHAPTER IV. 



THE COMPRESSED-AIR PROBLEM. 



The general problem of air-compression and of the ap- 
plication of compressed air to the re-development of power 



mg.l 



JE 



may be stated in simple terms. Fig. 
I really tells the whole story. The 
piston F is fitted to the cylinder E, 
so that we may assume it to move 
freely and without leakage. The 
piston being at A, as shown, and 
the cylinder being filled with air 
at a pressure of i atmosphere, and 
at normal temperature, a sufficient 
weight is placed upon the piston to 
force it down into the cylinder and 
compress the air contained in it to 
a pressure of, say, 6 atmospheres. 
The volume being inversely as the 
pressure, the piston should go down 
to C We find, however, that it ac- 
tually only goes down to B, and the 
reason is that while the air is being compressed the opera- 
tion of compression also heats it, and the heat distends or 
expands the air, and its volume is consequently consider- 
ably greater than it should be, upon the assumption that 
the volume is always inversely as the pressure. 

28 



c 



THE COMPRESSED-AIR PROBLEM, 29 

This is an illustration of the frequent differences that 
arise between theory and practice, with the usual result 
that practice is all right, and that theory will be in perfect 
accord with it when the theory in the case is complete. 
Theory thus far had not thought of the temperature of the 
air. 

Supposing both the piston and the cylinder to be abso- 
lute non-conductors of heat, and that the air heated by the 
compression loses none of its heat of compression, then if 
the weight which forced the piston down to B be taken 
away, the piston will be driven back to its original position 
at A, and the air contained in the cylinder will have re- 
sumed its normal pressure and temperature, and will have 
done as much work, or will have exerted as much force, by 
its return, as was employed in the act of compression. If 
while the piston was at B^ and with the weight upon it suf- 
ficient to balance the pressure of 6 atmospheres, the air 
by any means had been cooled to its original temperature, 
the piston would have fallen to C, and the law that the vol- 
ume varies inversely as the pressure would have held good, 
for then the initial and the final temperatures would have 
been the same. The air being thus cooled to its original 
temperature, and the piston being at C, upon removing the 
weight from the piston it would return only to Z>, instead 
of to A, When the piston arrived at D, the pressure of 
the air in the cylinder would have fallen to the original 
pressure of i atmosphere, and the piston at D would be 
balanced between the pressures above and below it. As 
the air is heated in the operation of compression, so is it 
correspondingly cooled during its expansion, and when the 
piston reaches D the air in the cylinder is then at atmospheric 
pressure, because it is then much colder than it was at the 



30 COMPRESSED AIR 

beginning ; and it is solely because of this loss of heat that 
the pressure falls so early, and that the piston does not re- 
turn to A where it started from. If while the piston is at 
D the air can by any means recover all the heat which it 
has lost, the piston will return to ^ as before. The dis- 
tance DA compared with CA^ or the distance DC^ repre- 
sents the total possible theoretical loss of power in the com- 
pression and the re-expansion of air. 

At the risk of anticipating a number of points that I 
hope to bring out more fully and in detail later on, we may 
now refer to the more or less practical diagram Fig. 2. 
This diagram, scale 40, is intended to show the practical 
possibilities in the use of compressed air at 75 lbs. gauge, or 
6 atmospheres. The line ab is the adiabatic compression- 
line, or the line of compression, upon the assumption that 
no heat is taken away from or is lost by the air during the 
compression. The initial temperature of the air being 60 
degrees, the final temperature would be about 415 degrees, 
and the final volume would be .28 of the original volume. 
The line ac is the isothermal compression-line, which as- 
sumes that all the heat of compression is got rid of just 
when it is produced, or that the air throughout the com- 
pression remains constantly at its initial temperature. The 
final volume in this case is .1666 of the original volume. 
Remembering that these lines, ab and ac^ represent the 
compression of the same initial volume of air, it is evident 
that there is quite a difference in the amount of power em- 
ployed in the two cases, and herein lies the loss, or the pos- 
sibility of loss, of power in the operation of compression. 
The mean effective pressure or resistance of the air for the 
stroke upon the adiabatic line abl is 35.36 lbs., while the 
mean effective pressure for the isothermal compression-line 



THE COMPRESSED-AIR PROBLEM. 



31 



ad'x'^ but 27 lbs., or only 76 per cent of the former. The 
comparison should, however, be reversed. The adiabatic 




mean effective pressure is 131 per cent of the isothermal 
mean effective pressure : 

27 • 35-36 : : I : i-3^ 

and this 31 per cent is, of course, the additional, or, as we 
might say, the unnecessary, power employed, assuming iso- 



32 COMPRESSED AIR. 

thermal compression to be attainable. Neither of these 
compression-lines, ab or ac^ is possible in practice. Air 
cannot be compressed without losing some of its heat dur- 
ing compression, so that the actual compression-line must 
always fall within or below the line ab. On the other 
hand, it is equally impossible to abstract all the heat from 
the air coincidently with the appearance of that heat, so 
that the actual compression-line must always fall outside 
or above the line ac. The best air-compressor practice of 
to-day is very near the line ao^ or the mean of the adiabatic 
and the isothermal curves. The actual line is generally 
above this, and seldom below it. It would be impossible to 
produce a line exactly coincident with this in practice. If 
we produced a line giving the same mean effective pressure 
as ao, it would probably run above ao for the first half of the 
stroke, and perhaps a little below it at the last. If the com- 
pression were two-stage or compound, — that is, if it were 
done in two or more cylinders instead of in one, — there 
would of course be breaks in the continuity of the com- 
pression-curve. The mean effective pressure for the line 
aol'x^ about 31.5 lbs., or still nearly 17 per cent in excess of 
the M.E.P. for the line acl. As aol represents exceptionally 
good practice, the loss of power for the average practice in 
air-compression, independently of the friction of the ma- 
chinery, may be put at 20 per cent. Lest some impatient 
ones should drop the subject here, or lest some rival of 
compressed air should pick up this and run away with it, 
we might insert a reminder here that all this loss is not 
necessarily final. In all these comparisons for efficiency 
the actual compression-line is always to be compared with 
the isothermal line acl, because that is the ideal line for 
compression without loss of power, and because the termi- 



THE COMPRESSED-AIR PROBLEM, 33 

nal volume cl is the volume actually available for use, no 
matter how economically or wastefully the air may have 
been compressed. Though at the completion of the com- 
pression stroke there is always some of the heat of com- 
pression remaining in the air, and its volume is always 
greater than cl^ that heat is always lost in the transmission 
of the air, or in its storage, and the available volume is 
never practically above cl. 

After the cooling and contraction of the compressed air 
comes the question of loss in the transmission of it. To 
cause the air to flow through the pipe there must be some 
excess of pressure at the first end of it, a constant decrease 
of pressure as the air advances, and consequently a loss of 
available power at the delivery end. But this loss has been 
greatly exaggerated. Here, as in other matters, the air-com- 
pressor builders have — unwittingly, we will say — done more 
harm than good as regards the interests of compressed air. 
Formidable tables are in all the air-compressor catalogues, 
showing the loss of pressure due to the friction of air in 
pipes. The tables are not dangerous, and are not published 
primarily for the purpose of frightening timid investors. 
They are only intended to suggest the size of pipe most 
suitable for any given case of transmission. If they tell us 
truly of the loss of pressure, they still fail to tell us that the 
loss of pressure is not necessarily, or to the same extent, a 
loss of power. The actual truth is that there is very little 
loss of power through the transmission of compressed air in 
suitable pipes to a reasonable distance, and the reasonable 
distance is not a short one. With pipes of proper size, and 
in good condition, air may be transmitted, say, ten miles, 
with a loss of pressure of less than i lb. per mile. If the 
air were at 80 lbs. gauge, or 95 lbs. absolute, upon entering 



34 COMPRESSED AIR. 

the pipe, and 70 lbs. gauge, or 85 lbs. absolute, at the other 
end, there would be a loss of a little more than 10 per cent 
in absolute pressure, but at the same tim.e there would be 
an increase of volume of 1 1 per cent to compensate for the 
loss of pressure, and the loss of available power would be 
less than 3 per cent. With higher pressures still more fa- 
vorable results could be shown. 

Having compressed the air and conveyed it to the point 
where we wish to use it, we may turn again to Fig. 2, and 
see what we will be able to do with the air. The air may 
be used in various ways with widely different economic 
results, and little ingenuity is required to accomplish enor- 
mous losses. Having the volume cl^ and using it in a cyl- 
inder of suitable capacity, cutting off so as to expand down 
to I atmosphere before release, the adiabatic expansion- 
line, or the lowest line that the air could make, would be 
the line cd^ and the total loss in the use of the air, as com- 
pared with the power cost of compressing it, would be the 
difference between the areas aolh and Icdh^ the latter being 
66 per cent of the former. 

The temperature of the air at c^ where the expansion be- 
gins, being assumed to be 60°, the cooling of the air which 
always accompanies its expansion will bring the tempera- 
ture far down the scale when d is reached, d being, of 
course, the end of the cylinder wherein the expansion takes 
place. The theoretical temperature of the air at the end 
of the stroke would be about — 150°. The actual tempera- 
ture in these cases is never found. as low as the theoretical 
temperature, because the air receives heat from the cylin- 
der and from the walls of the passages with which it comes 
in contact ; but it is usually still cold enough to cause seri- 
ous inconvenience in practice, and this cooling of the air 



THE COMPRESSED-AIR PROBLEM, 35 

may in many cases be fatal to its employment, entirely re- 
gardless of the economy of the case. The air almost inva- 
riably contains moisture, the amount varying with the sur- 
rounding meteorological conditions, and as the air becomes 
attenuated and so intensely cold the water is rapidly frozen 
in the passages, and soon chokes them up and stops the 
operation of the motor. The prevention or the circumven- 
tion of the freezing up of air apparatus is an additional 
complication of the compressed-air problem to be con- 
sidered later. 

The trouble from the freezing up naturally suggests the 
heating of the air before it is used. The heating or re- 
heating of the air, where it is practised, not only brings us 
out of our trouble about the freezing up, but it increases 
the volume of the air and its consequent available power at 
a very slight expense for the heating. If the volume of 
air cl^ being now at 60°, be passed through a suitable heater 
and its temperature raised to 300°, its volume will then be 
//, instead of cl^ or .2434 instead of .1666, an increase of 
volume of about 50 per cent. In practice, to insure a tem- 
perature of 300" in the cylinder at the beginning of the ex- 
pansion, it will be necessary to heat the air considerably 
above that temperature, say to 400"^, as the air loses its heat 
very rapidly. If now we use this reheated air, the volume 
cl then becoming //, and expanding this air down to e^ sup- 
posing the temperature at i to be 300°, the theoretical final 
temperature will be about zero. The actual temperature, 
it is pretty certain, will not be below the freezing-point, 
and all our trouble about the freezing of the passages will 
have disappeared, and the power realized will have been 
much increased. It seems to be quite practicable, by ef- 
fective cooling of the air during its compression, and by 



36 COMPRESSED AIR, 

reheating it before its re-expansion, to bring the expansion- 
line ie to enclose an area not less than that enclosed by the 
compression-line ao^ and then the entire losses will be those 
attributable to the clearances and to friction. It is said 
that in practice 85 per cent of the initial power has already 
been realized after transmitting the air to considerable dis- 
tances. " It is said '* accomplishes many wonderful feats. 

It was remarked above that the air after compression 
and transmission might be employed with widely different 
economic results. As an instance of *^how not to do it " I 
might cite the case, of too frequent occurrence, where air 
is delivered to a mine for operating rock drills and other 
mining machinery, and air then taken from the same pipe- 
line for operating a pump. This practice would be all 
right if the pump were adapted to the work to be done and 
to the pressure of air carried. The pump, however, is gen- 
erally a common direct-acting steam-pump, with all that 
the term implies, and which has been obtained without any 
reference to the economical use of the air. As it has prob- 
ably already been run by steam, or is designed to be run 
by steam, it calls for a low operating pressure ; this being 
a necessity on account of the condensation and loss of 
pressure in steam when transmitted through long pipes. 
Say that the compressed-air pipe carries 75 lbs. pressure, 
while the pump only requires 25 lbs. It would be an ad- 
vantage in a case like this to use a pressure-reducer in the 
supply-pipe at a considerable distance from the pump, so 
that the expansion to the lower pressure required might 
take place, and the air have an opportunity to recover its 
temperature and volume before going into the pump- 
This, however, is seldom attended to, and the required 
reduction of pressure is effected entirely by the throttle- 
valve at the instant of admission. The available power. 



THE COMPRESSED-AIR PROBLEM. 37 

then, when the air is so employed, will be represented by 
the area pmnh as compared with the area ablh^ or, at 
the best, aolh^ representing the power that was expended 
in compressing the air. Then, if we deduct the losses 
attributable to the useless filling of the large clearances of 
the common steam-pump, and to the leakages that are the 
usual accompaniment of such generally extravagant prac- 
tices, it is little wonder that compressed air is held in low 
esteem. Under circumstances far from the most unfavor- 
able I have found pumps realizing only 15 per cent of the 
power expended at the compressor, and I have no doubt 
that there are many pumps being operated, or whose oper- 
ation is attempted, where not more than 10 per cent of the 
original power is realized ; and, even then, when the use of 
compressed air for operating such pumps under such con- 
ditions is condemned, it is apt to be because they freeze up 
and won't go, rather than on account of their enormous 
waste of power. From the fact that a mining pump has a 
lift that is nearly constant, the pump, if properly propor- 
tioned and adapted to its work, should be an efficient mis- 
sionary for compressed air, rather than its most malignant 
traducer. 

The word " loss " that we find ourselves using in connec- 
tion with this general subject should not be allowed to 
mislead us. The use of compressed air is for the accom- 
plishment of a desirable purpose, and it is not to be ex- 
pected that such a purpose can be effected for nothing. 
The transmission of power is as much to be paid for as the 
generation of the power. Where water power is used, the 
means of transmission may be the principal item of cost. 
Where the difference between the power expended and the 
power realized is not excessive, that difference is simply a 
fair price paid for a good service rendered, and there is no 



38 



COMPRESSED AIR. 



loss about it. Where losses do occur in the use of com- 
pressed air, they are like the losses which occur in business, 
and which cut short many a brilliant career. Power is lost 
simply because it is not saved, and the means of saving are 
not hard to find nor far to seek. The excessive losses are 
not necessary nor unavoidable, nor without compensation. 
A failure to understand and appreciate this situation im- 
pedes the progress of compressed air. 

Table III. 

TABLE OF FINAL TEMPERATURES OF AIR COMPRESSED ADIABATICALLY 
TO DIFFERENT GAUGE PRESSURES FROM AN INITIAL PRESSURE OF 
I ATMOSPHERE, AND FROM DIFFERENT INITIAL TEMPERATURES. 



Final Pressure 


Initial Temp. 


Initial Temp. 


Initial Temp. 


Initial Temp. 


Gauge. 


0°. 


32°. 


60°. 


100°. 


I 


8 


41 


70 


Ill 


2 


16 


50 


79 


121 


3 


25 


59 


88 


132 


4 


33 


67 


97 


140 


5 


41 


75 


106 


150 


lO 


74 


113 


144 


191 


15 


104 


144 


177 


226 


20 


130 


171 


207 


258 


25 


153 


196 


233 


287 


30 


175 


219 


258 


313 


35 


195 


240 


280 


337 


40 


213 


260 


301 


360 


45 


231 


279 


321 


381 


50 


247 


296 


339 


401 


55 


262 


316 


357 


420 


60 


277 


328 


373 


437 


65 


291 


343 


389 


454 


70 


304 


358 


404 


471 


75 


317 


371 


419 


486 


80 


330 


384 


433 


501 


85 


342 


397 


446 


516 


90 


353 


410 


459 


530 


95 


364 


422 


472 


543 


TOO 


375 


435 


484 


556 


125 


425 


486 


540 


617 


150 


468 


532 


588 


669 


175 


507 


574 


633 


717 


200 


542 


612 


672 


781 



CHAPTER V. 

THE INDICATOR ON THE AIR-COMPRESSOR. 

READING AND COMPUTING THE DIAGRAM. 

The recent advances that have been made in steam- 
engine economy are not fully and generally realized. The 
engines of the Great Eastern steamship of forty years ago, 
representing the best engineering practice of her day, de- 
veloped 8000 horse-power. The engines of the Campania 
to-day show 30,000 horse-powxr upon practically the same 
consumption of coal. The gain is attributable to the 
adoption of the multiple expansion-engine, to the reheating 
between the steps, and to the general prevention of con- 
densation ; but the promoter and adviser all along the way 
through the successive stages of improvement has been the 
indicator. The indicator is to-day the companion and the 
trusted monitor of the steam-engine designer and builder 
as well as of the engineer. He would be a strange competitor 
for steam-engine trade in these days who would not freely 
and gladly show the cards from his engine, and it goes 
without saying that he would be an unsuccessful one. 
The air-compressor business is still an " infant industry," 
although a growing one. No better evidence is needed of 
the juvenility of the air-compressor trade of to-day than 
the difficulty of obtaining cards from most of the 

39 



40 COMPRESSED AIR, 

" Standard '' compressors. And yet the services of the 
indicator may be as valuable to the air-compressor and the 
air-engine as to the steam-engine, and they are certainly 
fully as applicable. 

All circumstances seem peculiarly to invite the applica- 
tion of the indicator to the air-compressor, and to the study 
of air-compression practice and results by its aid. In fact, 
the air-compressor seems to be the ideal and only perfect 
field for the indicator. So far as I know, a steam-actuated 
air-compressor is the only machine where an indicator can 
be applied and be made to tell the whole story of the 
pov^er developed and of the work done. In the steam- 
pump the report of the card of the water-cylinder is af- 
fected by questions relating to the inertia of the body of 
water. With a steam-engine of any type there is always 
some uncertainty about the friction of the working parts of 
the engine. We can take what we are pleased to call the 
"friction diagram,'* when the engine is running without 
doing any external work, and we know what resistance the 
steam has to overcome at that time ; but that tells us com- 
paratively little of the resistance of the engine parts when 
loaded. We know that the friction of nearly every work- 
ing part of the engine increases with the load, but when 
the load is on, we do not know from the indicator-card how 
much of its mean effective indicates actual work done or 
how much of it belongs to the friction of the engine, and 
to get the result with any certainty and accuracy it is nec- 
essary to employ some form of dynamometer in connection 
with the indicator, and let them fight it out between them. 
In the case of the air-compressor this is all different. The 
air-compressor is its own dynamometer. By taking cards 
from both the air- and the steam-cylinders at the same 



THE INDICATOR ON THE AIR-COMPRESSOR, 4I 

time, or when the compressor is running under the same 
conditions, we get a perfect statement of the power de- 
veloped and of the actual work done, and then we know 
too that the difference in indicated horse-power between 
the air- and the steam-cards clearly shows the power that 
has been expended merely to keep the machine in motion. 
The cards not only give the comparative total power and 
work, but also the relations of the one to the other at any 
point of the stroke, showing the air resistance at any point, 
as well as the force of the steam at the same point, and 
through this knowledge it will advise us whether the air is 
compressed with economy or whether better results are to 
be sought for. 

Realizing the importance of the indicator as an indis- 
pensable aid in the full development of economical air- 
compression, it is proper that we learn what we can of the 
peculiarities of the air-card and of the means of manipulat- 
ing and interpreting it. We can only consider at first the 
card from the single air-cylinder, in which the whole opera- 
tion of air-compression is completed at a single stroke. 
The cards from cylinders in which either stage of a com- 
pound compression is carried on assume peculiar shapes, 
which we may find pleasure in studying later on. 

To an indicator-man who has been brought up, as most 
have, exclusively upon steam-cards the air-card is at first a 
little confusing, from the fact that all the operations upon 
the one card are the reverse of those upon the other. The 
admission-line of the steam-card is the delivery-line of the 
air-card ; the expansion-line in the one is the compression- 
line in the other ; the exhaust or back-pressure line is the 
admission-line, and the compression-line becomes the re- 
expansion-line. One can, however, soon " catch on " and 



4^ COMPRESSED AIR. 

become familiar with each operation and its representative 
part of the diagram. 

It is not the purpose of this work to instruct in the ap- 
plication and use of the indicator. We must assume that 
the indicator is in competent hands, or its evidence will be 
worthless. Indicator-cards have, however, a way of telling 
for themselves frequently if they have not been taken with 
a reasonable regard for the essential conditions. As the 
peculiarly important part of the air-card is the compression- 
line, it is necessary that the drum movement be correct, and 
that, in proportion to its length, the travel of the card shall 
be accurately coincident with the piston travel at all points. 
Cards, to be relied upon, should not be taken until the com- 
pressor has been run long enough to have attained its com- 
plete working conditions. We know that the compression 
of air heats it, and that the heat then in the air is commu- 
nicated more or less to everything in contact with it. 
When the cylinder becomes heated, it has its effect back 
again upon the air, and until the compressor has been run 
continuously and at full pressure for an hour or so, the full 
temperature of the working parts has hardly been reached^ 
and the effect of the heated parts upon the temperature of 
the air at different points of the stroke will not be correctly 
indicated. Cards aken from a compressor that has only 
just been started will give a lower compression-line and a 
lower mean effective pressure (M.E.P.) than those taken 
after the cylinder and piston and connecting parts have 
been heated up to their mean working temperature. 

Fig. 3 is offered as an ideal and typical single-compres- 
sion air-cylinder card, designed to show the points and 
properties of the card, and the methods of manipulating 
and studying it. The card is somewhat smoother and 
cleaner and in most respects more perfect than any actual 



THE INDICATOR ON THE AtR^COMPRESSOR. 43 



card, except that the admission-line is purposely drawn 
rather low to keep it perfectly distinct from the atmosphere- 



I 

09 




line. The lines constituting the actual diagram are as 
follows : 

AB^ Compression-line 

BC, Delivery " 

CD^ Re-expansion '* 

DA, Admission '^ 



44 COMPRESSED AIR, 

These constitute the actual card, and together represent 
the complete cycle of operations occurring in one end of 
the air-cylinder for one complete revolution of the com- 
pressor-crank. The atmosphere-line, MN^ is also traced 
by the indicator, and is the neutral line of the diagram, or 
the line of departure in air-compression. 

For the proper interpretation of the diagram additional 
lines are to be drawn as follows : EF^ the line of perfect 
vacuum. This line is drawn parallel to the atmosphere- 
line, MN^ and at a distance below it determined by the 
scale of the diagram. The pressure of the atmosphere at 
sea-level being 14.7, and always decreasing as the altitude 
increases, the practice of calling the atmospheric pressure 
15 lbs. may be said to be a rather loose one. If the com- 
pressor is operated at a considerable altitude above the 
sea-level, as many are, the atmospheric pressure at the 
time and place where the diagram is taken should be ascer 
tained by a barometer, and the line EF be drawn accord- 
ingly. It should be remembered, as we will see when we 
get to it, that a height of only a quarter of a mile, or a little 
over 1300 feet, will make a difference of 7 per cent in the 
volume of air furnished. 

The vertical lines PA and CL having been drawn per- 
pendicular to MN^ and defining the extreme length of the 
actual diagram, the clearance-line GH m^.y next be drawn. 
This is drawn parallel to CZ, and the distance CG or LH 
may be ascertained by computation. The volume repre- 
sented by the rectangle APCL is the actual displacement of 
the piston for its whole travel. The volume of air acted 
upon by the piston is this volume increased by the volume 
CGHL remaining in the clearance-space of the cylinder. 
This volume of air, CGHL, at the end of the compression- 
stroke, and at the pressure indicated by the diagram, has upon 



THE INDICATOR ON THE AIR-COMPRESSOR, 45 

the return stroke of the piston re-expanded until it reached 
the atmospheric pressure again at D. This re-expansion is 
so quickly accomplished that whatever the temperature at the 
beginning the re-expansion is practically adiabatic. The 
relative volume before and after the re-expansion may be 
found in column 5 of Table IV. Assuming the scale of the 
diagram to be 30 and the pressure at CG to be 70 lbs. 
gauge, and designating LH by x we have the proportion 

X : DL -\- X \\ .288 : i 

Then the length DL being .25", we have 

X \ >2^-\- X \\ .288 : I ; 
then 

;:v = .072 + .288^, 
and 

.712JV = .072, 

X = .101. 

So that CG or ZH equals say yV"? ^^^ GIT may be drawn 
accordingly. 

Having drawn GIf, the rectangle APGH represents the 
total volume of air subjected to compression for the stroke, 
and noting the point a^ at which the compression-line be- 
gins to rise from the atmosphere-line, and drawing the 
perpendicular ae^ then aeGH represents the total volume 
of air at atmospheric pressure. The point ^, being the 
point at which compression from atmospheric pressure 
begins, may be considered the beginning of the whole 
diagram, and the cycle of operations for the entire stroke 
may be considered to start from this point. 

For computing the mean effective resistance the entire 
enclosed area of the actual diagram ABCD is to be taken, 



46 COMPRESSED AIR. 

and this area may be measured by the planimeter, or by the 
mean of a series of ordinates in the customary way, as with 
any other diagram. The area lying below the atmosphere- 
line of course represents the resistance upon the return 
stroke, but the diagrams from both ends of the cylinder 
being assumed to be similar, the entire area may be taken 
for the single stroke. The correct practice is to take dia- 
grams from both ends of the cylinder, and it should be 
followed if possible, but it is clearer and simpler for us 
here to consider only the single diagram. 

The M.E.P. of the diagram having been ascertained, the 
indicated horse-power (I.H.-P.) represented may be com- 
puted precisely as in the case of a steam-engine. Thus the 
M.E.P. in the diagram before us happening to be 30, if it 
were taken from a cylinder 20" dia. X 24" stroke at 80 
revolutions per minute, the I.H.-P. for the double stroke 
will be as follows : 

20"^ X .7854 X 30 X 4 X 80 -^ 33,000. 

I like always in such cases to put it down in this way, that 
I may be sure that I get in all the ingredients. It is not 
necessary to run for a table of squares or of areas, and no 
time is saved by doing so. The decimal .7854 is always 
cleanly divisible by the constant divisor 33,000, giving us 
.0000238. It is not difficult to remember this or to keep it 
posted with other labor-saving devices in a convenient place. 
The ciphers in the other factors will help us to elbow the 
decimal point to the right, and our case will then stand like 
this, a little string of simple and easy multiplications : 

2' X .238 X 3 X 4 X 8 = 

.238 X 384 = 91-39 I-H.-P. 
We will not here go into the question of the additions to 
be made to this for friction, etc. 



THE INDICATOR ON THE AIR-COMPRESSOR, 47 

The I.H.-P. having been ascertained, that gives us the 
power consumed, or the cost of the compression, and then 
we naturally want to know as soon as possible the actual 
quantity of air compressed and delivered, or how much we 
have got for our money. The indicator-diagram shows 
this very accurately. At the point ^, where the compres- 
sion-line takes its departure from the atmosphere-line, the 
cylinder is shown to be full of air at the atmospheric press- 
ure and corresponding density. This is not the whole 
cylinder, as a portion of it, Aa, has been already traversed 
by the piston. Whatever proportional distance the point 
a may be from the beginning of the stroke is to be deducted 
from the total length of the stroke and the remainder repre- 
sents the total actual volume of air at atmospheric pressure 
subjected to compression for that stroke. The compres- 
sion and delivery of the air goes on with the advance of 
the piston until it reaches the extreme end of its stroke at 
CZ, but when that is reached, the clearance-space LCGH 
is filled with air compressed, but not delivered, and upon 
the return of the piston this air re-expands until it reaches 
the atmosphere-line at ^, so that practically the travel of 
the piston from o Xo L and back again has accomplished 
nothing toward compression, and the distance oL is also 
to be deducted from the total length of the line AL^ when 
that line is taken to represent the volume of air compressed 
and delivered. In the diagram before us if AL be 3^' and 
ao be 3tV'> ^^ ratio of air compressed and delivered is 

— '^Z per cent of the cylinder capacity. As was re- 
marked, this diagram does not represent actual practice, 
and the ratio is not usually as low as this, being more fre- 
quently found hovering about 5 per cent in the best com- 
pressors, and rarely below that. 



48 



COMPHESSED AIR. 



So far as the indicator has anything to say about the 
economy of the air-compression,-and it has much to say 
-Its evidence is found chiefly in the compression-line of 
the diagram, and for comparison it is necessary to describe 
upon the diagram the theoretical isothermal and adiabatic 




Tsotliermdl Compression, 

curves. To facilitate the drawing of these lines the dia- 
grams F.gs^ 4 and 5 have been provided. The dimensions 
o he book have made it necessary to engrave these at one 
half of the full s,ze. They can readily be reproduced in 
full size by any draughtsman, and will be found useful for 
the purpose for which they were designed. As they stand 
here they are correct for scales that are double those indi- 



THE INDICATOR ON THE AIR-COMPRESSOR, 49 

cated. Thus the 15 ordinate is correct to apply to a 30- 
scale diagram, the 20 ordinate for a 40-scale diagram, etc. 
The compression-line of the air-card is more easily studied 
than the expansion-line of the steam-card, as it always has a 
definite beginning or point of departure at ^, such as the 




Adiahatic Compression 
Mff.5 

steam-card never has. From this point a the isothermal and 
the adiabatic curves are to be drawn. When the compressing 
piston is at ae, the air under compression includes the con- 
tents of the clearance-space at the farther end of the 
cylinder, and the total body of air under compression is 
represented by the rectangle aeGH. Vertical lines then 



so COMPRESSED AIR. 

are to be drawn dividing this space into 20 equal sections, 
and for convenience the lines are to be numbered, begin- 
ning at the line next to ae^ 19, 18, 17, 16, etc. It will not 
be necessary to number the last two or three lines to the 
right, or even to draw them, as the curves will not reach 
them. It will be noticed that the numbering does not 
include the boundary-lines ae and GH. Referring now to 
the diagram Fig. 5, for drawing the adiabatic curve, AB 
is the atmosphere-line and CD is the line of perfect 
vacuum, or the zero line of absolute pressure. Taking 
from the diagram, Fig. 5, the ordinate line correspond- 
ing to the scale of the indicator-card, the distance between 
AB and CD, measured upon this line, is the distance be- 
tween the atmosphere-line and the line of perfect vacuum, 
and the vacuum-line may be drawn upon the card accord- 
ingly parallel to the atmosphere-line and at this distance 
from it. Then upon the same vertical line of the diagram 
Fig. s the distance from AB to the first intersecting line 
above it indicates the distance to be laid oft* upon the ver- 
tical line No. 19 of the card as one point of the required 
adiabatic curve. The distance from AB to the second line 
above is the distance to be laid off upon the vertical line 
No. 18 as another point of the required curve, and so on : 
the points may be successively laid off upon the vertical 
lines of the card until the delivery-line BC is reached, or a 
little above it, when it is unnecessary to go further, and the 
required curve may be drawn coincident with the points that 
have been thus located. The isothermal curve may be drawn 
in the same way by the aid of Fig. 4. The points upon the 
first two or three vertical lines to the left may be so close to 
the actual compression-line of the card, or so nearly coinci- 
dent with it, that it is more confusing than helpful to draw 
the lines, and they may begin at a point further along the line. 



THE INDICATOR ON THE AIR-COMPRESSOR. 5^ 

This diagram, Fig. 5, assumes the atmospheric pressure to 
be 14.7 lbs., and is only applicable for approximately that 
pressure. If the atmospheric pressure for the altitude at 
which the compressor works, and where the indicator-card 
was taken, is decidedly less than 14.7, the atmosphere-line 
drawn by the indicator will represent that pressure and will 
not represent 14.7 lbs. The mean effective pressure can of 
course be computed by taking the area of the indicator- 
card as it stands; but if it is desired to draw the adiabatic 
and isothermal curves by the aid of our diagrams. Figs. 4 
and 5, it will be necessary to first draw a horizontal line 
representing the atmospheric pressure of 14.7 lbs. To do 
this first draw the zero line at a distance below the existing 
atmosphere-line corresponding with the ascertained atmo- 
spheric pressure and the scale of the diagram. Column i 
or 2 in connection with column 4 of Table IV., given at 
the end of this chapter, will generally furnish the data 
necessary for this service. The zero line having been 
drawn the sea-level atmosphere-line may then be drawn 
14.7 lbs. above according to the indicator-scale. When this 
line is drawn the point where the compression-curve crosses 
it may be noted and also the point where the reexpan- 
sion line strikes it, and ignoring the original atmosphere- 
line drawn by the indicator, the adiabatic and the isother- 
mal curves may be drawn precisely as previously discribed. 

These adiabatic and isothermal curves when described 
are rather an aid to the eye in making comparisons with 
the actual compression-line of the indicator-card than nec- 
essary in computation. The mean effective of the card is 
ascertained by the planimeter or by measurement, and the 
mean effective for adiabatic and isothermal compression un- 
der the same conditions may be found in Table II, and the 
economy of the actual compression, may be learned by com- 



52 



COMPRESSED AIR, 



parison with them. This paragraph is only meant to apply 
to approximately sea-level computations. 

Table IV will be found convenient in computations upon 
air-compression at various heights above the sea-level. 
Column 7 gives the values of the volumes of air actually 
compressed at any given height as compared with equal 
volumes of free air at sea-level. 



Table IV. 

TABLE OF ABSOLUTE PRESSURES, BOILING-POINTS, ETC., AT DIFFERENT 
HEIGHTS ABOVE SEA-LEVEL. 



. 


2 


3 


4 


5 


6 


7 










Weight 


Volume of 


Volume of 


Height 


Bar- 


Boiling- 


Absolute 


of I 


Air Equal 


Free Air at 


above 


ometer, 


point, 


Pressure, 
Lbs. 


Cu. Ft. 


to I Cu. Ft. 


Sea-level 


Sea-level, 


Inches of 


Degrees 


of Air at 


of Free 


equal to I Cu. 


Feet. 


Mercury. 


Fahr. 


60°, 


Air at 


Ft. at given 










Lbs. 


Sea-level. 


Altitude. 


O 


30 


212 


14.7 


.0765 


I 


I 


512 


29.42 


211 


14.41 


.07499 


1.02 


.98039 


1025 


28.85 


210 


14.136 


.07356 


1.04 


.96154 


1539 


28.29 


209 


13.86 


.07213 


1.06 


.9434 


2063 


27.73 


208 


13.587 


.07071 


1.08 


.9259 


25S9 


27.18 


207 


13.318 


.0693 


I. 10 


. 90909 


3115 


26.64 


206 


13.054 


.06793 


1. 12 


.89285 


3642 


26.11 


205 


12.794 


.06658 


I. 14 


.87719 


4169 


25-59 


204 


12.539 


.06525 


1. 17 


.8547 


4697 


25.08 


203 


12.289 


.06395 


1. 19 


.8403 


5225 


24.58 


202 


12.044 


.06267 


1.22 


.8197 


5764 


24.08 


201 


11.799 


.0614 


1.24 


.8064 


6304 


23.59 


200 


11.559 


.06015 


1.27 


.7874 


6843 


23.11 


199 


11.324 


.05893 


1.29 


•7752 


7381 


22.64 


198 


11.094 


.05773 


1.32 


.75757 


7932 


22.17 


197 


10.863 


.0565 


1.35 


.74074 


8481 


21.71 


196 


10.638 


.05536 


1.38 


.7246 


9031 


21.26 


195 


10.417 


.05421 


I. 41 


.7092 


9579 


20.82 


194 


10.202 


.05309 


1.44 


.6944 


10127 


20.39 


193 


9.99 


.05199 


1.47 


.6802 


10685 


19.96 


192 


9.78 


.0509 


1.50 


.6666 


11243 


19-54 


191 


9.57 


.0498 


1.53 


.6536 


1 1 799 


19.13 


190 


9.37 


.048S 


1.56 


.64102 


12367 


18.72 


189 


9.17 


.0477 


1.60 


.625 


12934 


18.32 


188 


8.98 


.0467 


1.63 


•6135 


13498 


17-93 


187 


8.78 


.0457 


1.67 


.6 


14075 


17.54 


186 


8.59 


.0447 


I. 71 


.5848 


14649 


17.16 


185 


8.41 


•0437 


1.74 


.5747 



CHAPTER VI. 
THE BEGINNING OF ECONOMICAL AIR-COMPRESSION. 

While it may easily appear that the purpose for which 
compressed air is used in any given case, or the conditions 
under which it is applied, make the question of power 
economy a distinctly subordinate one, and often relatively 
a very little and unimportant one, still it remains that for 
whatever purpose we use the air the cheaper we get it the 
better it is for us, and considerations of economy in the 
compression of it are always in order, and whatever saving 
is effected there is necessarily a clear gain. 

If we are to go into the compressed-air business " for all 
there is in it," the way to do it successfully and profitably 
is first of all to control the whole business. By this is not 
meant the establishment of a monopoly whereby we might 
have the compressing of all the air that is used, although 
there might be great profit in that. But with what air we 
do handle we cannot expect to accomplish much in the 
way of economy, unless we have as full control as possible 
of the air through each of the operations involved in its 
use : the compression of the air, its transmission to the 
place or to the apparatus where it is to be used, and its 
actual employment for the purpose intended. There are 
losses possible at several points, or at all points, along the 
series of operations, and there are commensurate savings to 

53 



54 COMPRESSED AIR, 

be effected by the avoidance of those losses. The losses 
may be defeated and the savings accomplished rather by a 
concentrated than by a divided control and accountability. 

Economy in air^compression should begin at the begin- 
ning, and at the beginning we first have to do with the 
** free air," or air at atmospheric pressure. This is our raw 
material, and it is of course desirable to get it as cheaply 
as possible. Now it so happens that in keeping our ac- 
counts of profit and loss in this business the raw material 
is measured out to us and charged against us not by 
weight, but by bulk, so that whatever air we want to use it 
is desirable to get it to the compressor in as small a volume 
as possible. The smaller the relative volume of air at the 
beginning of the series of operations the greater will be the 
profit at the end for any given service realized. The vol- 
ume of free air increases or diminishes as its temperature 
rises or falls, which means that we should get our free air 
as cold as possible. The colder the air is the less it will 
measure in cubic feet, and we may consequently say that 
the colder it is the cheaper we are getting it. 

It seems necessary in all of the operations with com- 
pressed air to keep the accounts of profit and loss, and the 
record of work done, by the volume of free air that is 
handled. This involves fewer uncertainties than if we 
were to base our computations upon the volume of air after 
compression to any given pressure, or at any later stage in 
its transmission or use. The air-compressor, when the 
necessary corrections for clearance, etc., have been deter- 
mined, is a very reliable air-meter, and from it may be ob- 
tained a very close record of the free air taken in by it and 
compressed and delivered. After the beginning of opera- 
tions the temperature of the air is such an uncertain and 



BEGINNING OF ECONOMICAL.^ AIR-COMPRESSION, 55 

variable factor, and is still of such importance in the result, 
that all calculations are upset by it. The absolute meas- 
ure of the air operated upon would of course be its weight, 
but this it is not possible to ascertain in extensive practical 
operations. 

The volume of air at common temperatures varies di- 
rectly as the absolute temperature. With our air-supply at 
60° its absolute temperature is 521,° and the volume of it 
will increase or decrease -^\-^ for each degree of rise or fall 
of temperature. In securing our supply of free air for the 
compressor, then, if we can get a difference in our favor of 
5° by laying a pipe and leading the air in from the outside 
of the compressor-room, or from the shady side of the 
building, or from the coolest place near by, instead of 
using the air in the compressor-room, we accomplish a sav- 
ing of about I per cent. If we secure a difference of 
temperature of 10°, which in practice is frequently quite 
possible, we save 2 per cent absolutely without cost, ex- 
cept the first cost of the pipe or box to lead the air in. I 
know that the average machinist or engineer, or the man 
who calls himself distinctively the practical man, cannot 
commonly appreciate these small figures, or have any re- 
spect for such small savings, but when it comes to business 
I do not know why they should not have the same weight 
as the same values have in any other of the details of busi- 
ness. Brokers have to live and flourish upon commissions 
of \ or yi^ of I per cent, but " practical '* men are so 
wealthy that i or 2 per cent is not worth considering. 

The pipe to convey the cool, free air from the point 
where we determine to take it to the compressor may as 
well be of wood or of cement or earthenware as of iron, 
and in fact such material for its non-conductivity is to be 



56 COMPRESSED AIR, 

preferred. The pipe should of course be large enough to 
convey the required flow of air with perfect freedom. 
Some of the best air-compressors of the day may be con- 
nected quite readily with an outside air-supply, and they 
make provision for it ; others cannot easily be so con- 
nected, which is unfortunate for them. 

Another point that has not hitherto received the atten- 
tion that it deserves, although much more important than 
the preceding, is the necessity, in the interest of the best 
power economy, of not only getting the air as cold as pos- 
sible ai the compressor, but of getting it as cold as possible 
into the compressor. We have too readily assumed that 
the one covers the other, when, as a matter of fact, it never 
does. The temperature of the air at the cylinder and about 
to enter it does not guarantee the temperature of the air in 
the cylinder at the moment when the cylinder is filled and 
compression begins. It is not too much to say that the 
temperature of the air outside the cylinder and of that 
inside is never the same. Yet it is not to be forgotten that 
the sole object of the effort to get cool air for the compres- 
sor is to have it as cool as possible, and of as small a volume 
as possible, at the moment when compression begins. How 
cool the air may have been at any previous moment, how- 
ever near, has nothing to do with the case. 

In another chapter I have remarked that the air-compres- 
sor is the ideal and the only perfect field for the use of the 
indicator, that it is the only place where the indicator dia- 
grams will tell the whole story both of the power expended 
and of the work accomplished. This is undoubtedly true, 
but it is a statement that is quite likely to be understood to 
say more than it does say. The indicator diagram from 
the air-cylinder does not tell all that it seems to tell, or it 



BEGINNING OF ECONOMICAL AlR-COMPRESSION, S7 

tells it wrong. You may note upon the diagram the point, 
very near the beginning of the compression-stroke, where 
the cylinder, if we may believe the diagram, is filled with 
free air, or air at atmospheric pressure, and from that, after 
deducting what fills the clearance-space at the end of the 
stroke, we may compute the volume of free air actually 
compressed and delivered ; and then, later, we may realize 
that we have not got the volume of free air that the dia- 
gram testifies to. This is due to the fact that the diagram 
has nothing to say about the actual temperature of the air, 
either at its admission, at its discharge, or at any point of 
the stroke. With steam, unless it is superheated, the press- 
ure indicated guarantees the temperature; with air the 
pressure and the temperature have no necessary connection. 
I may show you a diagram from an air-compressing cylin- 
der where the air-admission line is almost exactly coin- 
cident with the atmosphere line, and where the compression 
pne begins to rise above the atmosphere line immediately 
at the beginning of the compression-stroke, showing that 
the cylinder is completely filled with air at atmospheric 
pressure, and we may congratulate ourselves that the dia- 
gram is an excellent one in this respect ; but suppose that 
when the cylinder is just filled, and compression is just 
beginning, our cylinder is filled with air at 120° instead of 
at 60°, which is the temperature of the supply. It means 
that our cylinder holds rather less than .9 of the air that 
we are assuming that it holds, and which the diagram says 
that it holds. It means not merely that the practical capa- 
city of the compressor is one-tenth less than we assume it 
to be, but that for the compression of this nine-tenths we 
are still expending the full power as represented by the 
steam-card. If the difference in indicated horse-power 



58 COMPRESSED AlR, 

between the air-cylinder and the steam-cylinder is ten per 
cent of the air-cylinder, or if the power ratio of the steam 
to the air be i.i : i, it is not a bad showing. This is about 
the ratio obtained in the best air compressors of the day. 
But if this I.I, the power of the steam-cylinder, is to be 
compared not with i, the full capacity of the air-cylinder, 
but with .9, its actual contents, the case is quite different: 
.9 : I.I : : I : 1.22, which is a result not worth bragging 
about by any compressor builder. 

There seems to be no means of ascertaining the actual 
temperature of the air during the operation of compression. 
The temperature of the air at different points of the stroke 
would be easily computable from the indicator diagram, 
which shows the pressure attained at any point, if we only 
knew the initial temperature, but as we have no means of 
knowing the initial temperature we do not know the actual 
temperature at any time. Who will tell us how to find 
it out ? This does not seem to be an impossible problem. 
It looks at first sight almost as simple — not quite — as to tell 
how fast a stream of water flows through a pipe. But no- 
body has yet invented a satisfactory water-meter. In the 
meantime we can only use our mechanical judgment and 
common sense as to the best means of getting the air into 
the cylinder as cool as possible. We can say in a general 
way that the air should enter the cylinder by the shortest 
and most direct possible passage, and with as little contact 
as possible with any metal at a higher temperature than its 
own. 

Some interesting matter bearing upon the topic we are 
speaking of is found in a paper upon ** Blowing Engines,'' 
by Mr. Julian Kennedy of Pittsburgh, read before the 
Mining Engineering Division of the World's Engineering 
Congress at Chicago. Mr. Kennedy says: 



BEGINNING OF ECONOMICAL AIR-COMPRESSION. 59 

*' This heating of the incoming air expands it, and pro- 
portionately reduces the weight of air entering the cylinder 
at each stroke. I have observed this in the case of an 
engine which was so constructed as to cause the air to 
travel about 3 inches over the hot metal in thin films -f-/ 
thick. Alongside of it was another engine of the same size 
and make, except that valves were used which allowed the 
air to pass over about i inch of metal, the openings being 
of such size that each stream of air was 2 inches in thick- 
ness. Careful and repeated tests of these engines, when 
both were in good order, showed that, while the indicator 
diagrams were practically the same, the one with the large 
valves would burn about 10 per cent more coke in the fur- 
naces, a result which could only be explained on the sup- 
position that, in the case of the engine with the small air 
openings, the incoming air, in passing through the small 
and tortuous passages in the heads, was heated about 25° C. 
more than in the case of the other engine." 

The above, it should be remembered, speaks only of blow- 
ing engines, where the air-pressures are low, and where the 
heat of compression and the heating of the parts in contact 
with the compressed air do not range high. In an air-com- 
pressor every part of the cylinder in contact with the air 
after compression naturally becomes much hotter than in 
the blowing-engines that Mr. Kennedy speaks of, and the 
heating of the inrushing air may also be much greater. 

The air remaining in the clearance space of the air-cylin- 
der at the end of the compression-stroke, being between 
the hot piston and the more or less heated cylinder head, 
may not have lost much of its heat of compression, but by 
the cooling action of the water-jacket it must have lost some 
of its heat, and its temperature cannot therefore be as high 
as the theoretical temperature due to the compression. 



6o COMPRESSED AIR, 

Still it is comparatively hot, and when it is remembered 
that this hot air becomes a part of the next cylinder full of 
air to be compressed it has been assumed that therefore the 
mean temperature of the contents of the cylinder is some- 
what increased by this admixture. But this conclusion is 
hasty and unwarranted. This hot air in the clearance- 
space is only hot when under the terminal pressure, and as 
at this pressure it is not as hot as the theoretical tempera- 
ture for the given compression it cannot upon its re-expan- 
sion to atmospheric pressure be as hot as it was before its 
previous compression began. It must be really somewhat 
cooler than the air that rushes in to fill the cylinder for the 
next stroke, and it therefore does not contribute any heat to 
the new charge of air, but rather receives some heat from it 
and slightly cools it. 

The air remaining uncompressed in the clearance-space 
at the end of the compression stroke, as it does not raise 
the temperature of the incoming air or tend to increase its 
volume, has therefore no bad effect in that respect, and in 
no way increases the power required for compressing a 
given quantity of air. The power that has been expended 
in the compression of this air in the clearance-space is not 
lost, or but a portion of it, as it gives out in its re-expan- 
sion, by helping the piston upon its return stroke, most of 
the power expended in its compression. Clearance in the 
air-cylinder, therefore, represents a loss of capacity in the 
air-compressor rather than a loss of power. And it is on 
account of its reducing the capacity of the compressor to 
compress its full quota of free air per stroke that it is 
desirable to keep the clearance as small as possible. 



CHAPTER VII. 
OF COMPRESSION IN A SINGLE CYLINDER. 

Proceeding now to look into the actual conditions of 
practical air-compression, and the possible economy to be 
attained, it is perhaps most proper to consider the perform- 
ance of the best compressors in actual use rather than the 
ideal, and perhaps in some respects the practically impos- 
sible, compressor. The air-compressors now most gener- 
ally in use have horizontal, double-acting air-cylinders 
more or less completely water-jacketed, and with various 
devices for heads, valves, pistons, etc. The entire com- 
pression is effected at a single operation and the pressure 
of the air usually ranges from 60 to 80 lbs. gauge. Whether 
these compressors prevail through the operation of the law 
of natural selection and the survival of the fittest we may 
not rashly say ; while they may not exhibit the highest 
attainable economy in the compression they are found to 
require little looking after, cost little for repairs, are gener- 
ally reliable, and in the long run they are found to pay. 

Supposing that we are filling the air-cylinder by the 
natural inflow of the air under the pressure of the sur- 
rounding atmosphere, and that we have got into the cylin- 
der the greatest possible actual weight or quantity of air 
under those conditions, which means that our air is little, 
if any, below the density of the surrounding air from which 

61 



62 COMPRESSED AIR. 

it is drawn, and, assuming that the air is also as cool as we 
can get it, we may then be said to have got our material as 
cheaply as possible, to have started our business under the 
most favorable conditions, and with encouraging prospects ; 
and we may then, and not until then, consistently and 
without reproach look for the available means of economy 
in the actual operation of compression. The same con- 
siderations that tend to economy in the procuring of the air, 
or of getting it into the cylinder, hold good also in all the 
subsequent operations of compression. The smaller the 
bulk or volume of any given quantity or weight of air the 
cheaper can the compression be effected and the better 
will be the economy ; and, as the volume of the air at any 
given pressure depends upon its temperature, the supreme 
consideration throughout the operation is to keep the air 
as cool as possible. The question of temperature is the 
important one to be kept constantly in sight, and its im- 
portance resides entirely in its effect upon the volume of 
air operated upon. While, as we know, practical air-com- 
pression has not as yet come down to the 'minute econo- 
mies, where eventually the profits of legitimate business are 
to be sought, still the losses that are possible in compres- 
sion, and the gains that are to be effected by avoiding or 
overcoming those losses, have received more or less atten- 
tion from the compressor builders. 

It is w^ell enough understood that, in the interest of 
power economy, the air should be kept as cool as possible 
at every stage of the compression, and the earlier the cool- 
ing is effected the greater is the gain, as all of the subse- 
quent operation is more or less affected by it. Keeping 
the air cool during compression means actually cooling the 
air during compression. No compression can be effected 



OF COMPRESSION IN A SINGLE CYLINDER, 63 

without a corresponding rise of temperature in the air com 
pressed. "YX-i^ox^iicdX^y the rise will always be the same 
where the conditions are identical. Starting with a given 
volume of air and with the air at a given pressure and tem- 
perature, and compressing to another and higher pressure, 
the resulting volume and temperature should always be the 
same. Practically the temperature of the air after com- 
pression, or during compression, is never as high as the 
theoretical temperature, or as high as the books and tables 
say that it should be, and it is also widely variable under 
apparently slight changes of conditions. This is not at all 
because the theory in the case is incorrect, but rather that 
it is incomplete, in that it is not cognizant of all the condi- 
tions that affect the case. Theory says, and correctly, that 
the element of time has nothing to do with the heat of 
compression ; that a given volume of air when compressed 
to another given volume will have its temperature raised so 
much, whether it takes a minute, an hour, or a week to do 
it. Practically time has a great deal to do with the case. 
The readiness with which the air will receive heat from or 
impart it to whatever may be in contact with it, and the 
small amount of heat actually represented by its changes 
of temperature render the actual volume a highly elusive 
quantity, and time becomes a playground for it. 

In a compressing-cylinder in actual use all the parts of 
it, the body of the cylinder, the heads, the piston and rod, 
the valves and seats or guides become heated by their 
contact with the compressed air ; but while they are thus 
becoming heated they are only heated by this contact, and 
while being heated they are also being cooled, as they are 
constantly transmitting some of the heat received from the 
air and dispersing it by conduction or radiation ; and, con- 



64 COMPRESSED AIR, 

sequently, these parts are never as hot as the air that heats 
them — when the air is at its hottest — and the air also is 
not as hot as it would have been but for its contact with 
them. The metallic parts after a time of continuous opera- 
tion attain an average temperature, and will not get any 
hotter. The mean temperature attained will depend upon 
the facilities provided for taking the heat away. Nothing 
better is known or has been suggested for conveying away 
the heat than cold water. It is now the general practice to 
make the shell of the cylinder double with a water-space 
between the cylinder proper and the outer shell, and, where 
the style and arrangement of the valves permit, the heads 
also are made hollow, with water circulating in them. 
Water has also in some cases been circulated in the body 
of the piston. These arrangements undoubtedly help to 
reduce the mean temperature of the parts and to make 
them more effective in cooling the air. 

When the entire compression is effected in a single 
cylinder the heat of compression is abstracted from the air 
mostly at the latter part of each stroke, when the air is at 
its hottest and when the difference in temperature between 
the air and its surroundings is the greatest. Indeed it is 
to be supposed that in active compression the air loses 
none of its heat of compression during the earlier part of 
the stroke unless the means of cooling the cylinder parts 
are unusually efficient and operative. If at the beginning 
of the stroke the cylinder is hotter than the air, as it natu- 
rally must be, the air is naturally heated rather than cooled 
by the contact. Practical evidence of this is not wanting. 
Indicator diagrams from air-compressing cylinders are easily 
to be found, as Fig. 6, where the compression-line of the 
diagram does not leave the adiabatic line until the first 



OF COMPRESSION IN A SINGLE CYLINDER, 65 

quarter of the stroke is traversed. In this connection it 
may be remarked that for evidence upon the point that we 
are considering any indicator-cards that are taken when a 
compressor has just been started, and before the cylinder 



parts have attained their full average temperature, are not 
not be considered. • Such cards promise better than the 
actual performance of the compressor will fulfil. 

The heating of the air does not continue throughout the 
whole stroke of the piston, but is accomplished and ceases 
at the moment that the full pressure is reached ; and for the 



66 COMPRESSED AIR, 

remainder of the stroke, while the compressed air is being 
ejected from the cylinder, the air is becoming somewhat 
cooler, while the metal inclosing it is becoming hotter. 
The heat of the cylinder parts is not evenly distributed. 
The ends of the cylinder and the entire cylinder-heads, 
being exposed to the air when it is hottest, naturally be- 
come hotter than the middle of the cylinder, which never 
feels the hottest air. The importance of the water-jacket, in 
the absence of any better cooling device, is obvious enough. 
The cooling effect of the water is greater when it is applied 
to the cylinder-heads than anywhere else, because they are 
exposed to the heated air for the greater portion of the 
stroke, while the inner surface of the cylinder itself is cov- 
ered by the advancing piston. Apart from the cooling of 
the air under compression, and the reduction of its volume, 
the water-jacket is a necessity as affecting the lubrication 
of the cylinder surfaces. Without some such means of 
cooling the cylinder it would become so heated as to burn 
the oil and render it useless as a lubricant. 

As the ultimate object of the water-jacket is the saving 
of power, by the reduction of the volume of air under com- 
pression, it is an interesting question as to what is practi- 
cally accomplished by it. What cooling of the air is 
actually effected and what saving of power is accomplished 
by complete water-jacketing ? From all that I have been 
able to observe I think that we may say that when com- 
pressing in a single cylinder to from 60 to 80 pounds gauge- 
pressure, and at a piston-speed not exceeding 300 feet per 
minute, one half of the total possible cooling is all that 
may be expected to be accomplished. This, I think, may 
be done, although I will not undertake at this writing to 
show where such a performance is actually to be found. 



OF COMPRESSION IN A SING IE CYIINDER. 6j 

If by a single compression we can produce a compression- 
line midway between the adiabatic and the isothermal 
lines we are leaving but a narrow margin for further saving ; 
and if that saving is to be accomplished by complications 
of mechanism, by increased friction and clearance losses, 
and by additional cost of maintenance, it will be but a 
doubtful gain. 

The device of cooling the air by the injection of a spray 
of water into the cylinder is probably the most effective 
cooling arrangement that has ever been devised, but col- 
lateral objections have driven it completely out of use, in 
all new compressors at least, in the United States. When 
the spray is used the success of it as an air-cooling agent is 
entirely dependent upon the mode of its application. The 
spray can only possibly effect the intended purpose when 
diffused through the air while it is being compressed, or 
during the compression-stroke of the piston. It can only 
cool the air while it is hot, or while it is being heated ; so 
that to admit the water with the incoming air is only to let 
it fall inert and useless to the bottom of the cylinder, to be 
driven out by the piston. Air so admitted may have a 
quasi usefulness in filling the clearance-space at the end of 
the stroke, but it can do little or nothing toward cooling 
the air. The presence of the water may also make it un- 
safe to run the compressor at a speed that would be other- 
wise safe and proper. With the use of water in the com- 
pression-cylinder, whether properly injected or not, no 
satisfactory means of lubricating the surface of the cylinder 
has ever been found, so that the friction of the piston and 
the loss of power by that means is greater than with other 
systems of compression. The piston and cylinder surfaces 
also wear away rapidly, so that the repair cost and incon- 



68 



COMPRESSED AIR, 



venience is greater than with other systems. While there 
is no compressor-builder, that I know of, who is now offer- 
ing a compressor furnished with injection-pumps, there 




is no objection to any builders retaining in their catalogues, 
as they do at this writing, the standard arguments against 
the injection system, because it helps to give the catalogue 



OF COMPRESSION IN A SING IE 'CY UNDER, 69 

a formidable appearance, you know, and no one is harmed 
by the practice. 

The size of the compression-cylinder is a thing to be 
thought of in the consideration of economical air-compres- 
sion. Other things being equal, a cylinder of small diame- 
ter has a decided advantage over a large one in cooling the 
air during compression. In a large cylinder the portion of 
air immediately in contact with or lying near to its water- 
cooled surfaces will be cooled by the contact, but the air 
in the middle of the cylinder will be little and slowly 
aifected. A number of small compressors will show better 
results, as regards the cooling of the air, than a large com- 
pressor can show. This has something to do with an indi- 
cator-diagram that I now have the pleasure of offering 
(Fig. 7). I have no hesitation in saying that it is the best 
and most satisfactory diagram made by a single compres- 
sion that I have ever seen. The scale of the diagram is 
30. It was taken from one of a series of small compression- 
cylinders entirely submerged in water. The speed, 96 revo- 
lutions, was not slow, so that the result was remarkable. 
This diagram at least shows conclusively the possibility of 
compressing in a single cylinder with the compression-line 
well within the mean of theoretical adiabatic and isothermal 
compression. 



CHAPTER VIII. 

TWO-STAGE AIR-COMPRESSION. 

What may be called the common working-pressure for 
compressed air, or the pressure at which the air is most fre- 
quently used, is from 60 to 80 lbs. gauge, or say 75 lbs., or 
6 atmospheres. This is the usual pressure employed in 
operating rock drills, hoisting-engines, pumps, and the gen- 
eral line of mining, tunnelling, quarrying, and rock-excavat- 
ing machinery, and this is even now the largest general field 
for the use of compressed air. While most of the com- 
pressed air that is used is compressed in single air-cylin- 
ders, usually double-acting, each cylinderful of free air 
being compressed and delivered by each single stroke of 
the piston, some of the air is compressed by two-stage com- 
pressors, or. by compound compression, and most theorists 
advocate the two-stage compression system for ordinary 
pressures ; and, as a matter of fact, the two-stage compres- 
sors maintain a respectable position among the various 
competitors. For high pressures two-stage or triple or 
even quadruple compression may be necessary, but for the 
pressures that are commonly employed, at least up to 6 
atmospheres, the ultimate economy of two-stage compres- 
sion is still an open and debatable question. 

When we come to look into two-stage or compound com- 
pression, we find a number of interesting points to be con- 

70 



TWO-STAGE AIR-COMPRESSION, 



71 



sidered, and the air-compressing problem becomes more 
complex. Th^ conditions in detail involved in the opera- 
tion of two-stage compression are perhaps better exhibited 
where the cylinders are single-acting, and that style of 
compressor we will first consider. I offer now — Figs. 8, 9, 
and 10 — a set of indicator-diagrams, scale 80, from the air- 




Tig,SO 



cylinders of a two stage compressor. The cylinders of the 
compressor from which these cards were taken were each 
single-acting arranged tandem, the two pistons upon the 
same piston-rod, and doing the work of the alternate cylin- 
ders upon the alternate strokes of the engine, the steam- 
cylinder also being in line with the air-cylinders and actu- 
ating the same piston-rod. The cylinders were 20" and 
I if" in diameter respectively, and the stroke 18". The 
capacity ratio of the two cylinders, deducting the area of 
the piston-rod in the larger cylinder, was i : .35. Cards 
were taken from both air-cylinders with the compressor de- 



72 COMPRESSED AIR, 

livering air at 35 lbs., at 40 lbs., and then by intervals of 
10 lbs. all the way up to 120 lbs. The cards here pre- 
sented are as good as a greater number for bringing out 
the peculiarities of the case. Fig. 8 is from the first or low- 
pressure cylinder. This card did not vary in any particular 
throughout the whole series from 35 lbs. to 120 lbs., and 
it would have continued the same no matter how high the 
terminal or delivery pressure of the second cylinder were 
carried. A tracing was made of one of these cards and 
laid over several others of the series, and the variation was 
so slight as to be scarcely discoverable at any point. 

The mean effective pressure of Fig. 8 is 15.8 lbs., and 
the terminal pressure is 35 lbs. While the terminal press- 
ure in this first cylinder is 35 lbs., it does not mean that 
if the two-stage compressor were compressing and deliver- 
ing air at 35 lbs. gauge, the first cylinder would be doing all 
the work of the compressor. It is to be remembered that 
the complete work of air-compression comprises two dis- 
tinct operations : the compression of the air to the re- 
quired pressure, and the expulsion or delivery of the air 
against practically the same pressure in the air-pipes or in 
the air-receiver. In the case that we are now considering, 
where the air is delivered from the compressor at a press- 
ure of 35 lbs., the first cylinder happens to do all of the 
work of compression, and none of the work of expulsion or 
delivery. In any case of two-stage compression, if either 
cylinder is to be called distinctively the ** compressing" cyl- 
inder, that term always belongs to the first cylinder rather 
than to the second. If our two-stage compressor were de- 
livering air at a pressure higher than 35 lbs., the first cylin- 
der would still compress the air to 35 lbs. as before, or 
would do only a portion of the total compression, and of 



TWO-STAGE AIR-COMPRESSION, 73 

course none of the delivery. The height to which the first 
cylinder will continually compress the air is determined 
by the relative capacities of the two cylinders modified to 
some extent by the cooling of the air that may be effected 
in its passage from one cylinder to the other. The work 
of the second cylinder when the compressor is delivering 
the air at 35 lbs. is shown by Fig. 9, taken from that cylin- 
der. The delivery-line ba in this case would be a per- 
fectly horizontal line if the movement of the piston were 
uniform throughout the stroke, the rise and fall of the line 
corresponding approximately to the acceleration and re- 
tardation of the piston. 

At whatever pressure the compressed air may be deliv- 
ered by the compressor the mean effective pressures for 
the two distinct operations of compression are never alike. 
The mean effective pressure for compression only is al- 
ways lower than the M.E.P. for delivery only, and of 
course also lower than for the combined operation of com- 
pression and delivery as performed in a single cylinder. 
In the compression table II. columns 6 and 7 give the 
mean effective pressures for the whole stroke when all of 
the work of compression and delivery is done in a single 
cylinder, column 6 being for isothermal and column 7 being 
for adiabatic compression. In the same table columns 8 
and 9 give respectively the isothermal and the adiabatic 
M.E.P. for the compression part only of the stroke of a 
single air- cylinder. 

Resuming now our compound compression, and referring 
again to Fig. 8, we notice that its mean effective pressure — 
15.8 — is greater than the pressures given in either columns 
8 or 9 for compression only to 35 lbs., where the entire 
work of the compressor is done in a single air-cylinder. 



74 COMPRESSED AJR, 

The table referred to, as we have previously stated, has 
nothing to do with compound compression, but the com- 
parison of figures might provoke a suspicion that in com- 
pound or two-stage compression we are doing the same 
work of compression as in the single air-cylinder, but at 
greater expense, and it is therefore proper to refer to it 
here. The case represented is different in more than one 
particular. In single-stage compression the compression is 
all done in the one cylinder, and throughout the entire 
compression-stroke the same quantity or weight of air is 
acted upon. In Fig. 8 we are not doing the entire com- 
pression part of the work in the one cylinder, although it is 
begun there, and the weight of air acted upon is not the 
same throughout the stroke. While at the beginning of the 
stroke the air acted upon is the free air contained in the 
first cylinder and just admitted from the atmosphere, this 
continues only for the first half of the stroke, and for the 
latter part of the stroke the whole body of air then undergo- 
ing compression consists not only of all the contents of the 
first cylinder that have not been expelled by the advancing 
piston, but also of the entire contents of the passage con- 
necting the two cylinders, and the contents of that part of 
the second cylinder which has been vacated by its retreat- 
ing piston. Fig. 8 shows the compression beginning at a, 
with the beginning of the stroke, and with the free air con- 
tents of the first cylinder alone. This goes on until the 
point is reached, near the middle of the stroke, and then 
communication is opened with the air-passage that connects 
the cylinders, and through that with the second cylinder. 
When the previous compression-stroke of the first cylinder 
ended, the passage connecting the cylinders was filled with 
air compressed to 35 lbs., and by the action of the valves 



rWO'STAGE AIR-COMPRESSION, 7$ 

this passage was then for a time shut off from communica- 
tion with either cylinder. This passage, in fact, remains 
shut off from communication with either cylinder during 
the whole of the return stroke, while the first cylinder is 
being filled with a fresh charge of free air, and while the 
compressed air in the smaller cylinder is being expelled 
into the discharge-pipe and the air-receiver. When the 
return or intake stroke of the larger cylinder has ended, 
which return .stroke is the delivery-stroke of the smaller 
cylinder, and when the compressed air has all been expelled 
from the smaller cylinder by its piston reaching the end of 
it, then the return stroke of the smaller cylinder commences, 
this stroke being of course coincident with the next com- 
pression-stroke of the larger cylinder. With the com- 
mencement of the return stroke of the smaller piston the 
air confined in the connecting, passage begins to re-ex- 
pand and to flow into the smaller cylinder. The pressure 
is thus falling in the air-passage, on account of its supply- 
ing the smaller cylinder, and at the same time compression 
is going on in the larger cylinder, and the pressure in it is 
rising. These simultaneous operations go on until at 
length the point o is reached, where the pressure in the 
larger cylinder exceeds the pressure in the air-passage and 
in the smaller cylinder, and the air from the larger cyl- 
inder begins to flow into the air-passage, and at the same 
time the entire contents of the air-passage and of the 
smaller cylinder become constituent parts of the body of 
air that is being compressed by the advancing piston of 
the larger cylinder, and thereafter until the end of the 
stroke the compression of the combined contents of large 
cylinder, air-passage, and small cylinder goes on together. 
The last one third of the compression-stroke in Fig. 8 and 



'J 6 COMPRESSED AIR. 

nb in Fig. 9 or 10 represent the same operation of com- 
pression, the line in Fig. 8 showing a somewhat higher 
pressure than in Fig. 9 or 10 on account of the friction to 
be overcome in passing the valves and passages. 

The mean effective pressure for the combined operation 
of compressing and expelling the air at 35 lbs., or for the 
whole operation of air-compression so termed, when per- 
formed adiabatically in a single cylinder is, theoretically, 
21.6 lbs. Practically, without any special arrangements for 
cooling the air, the M.E.P. usually falls somewhat below the 
above figure, as the air inevitably loses more or less of its 
heat during the operation. If we consider Fig. 8 in con- 
nection with Fig. 9, they together represent the whole op- 
eration of compression to 35 lbs. by two-stage compression, 
Fig. 8 representing the compression of the air and Fig. 9 
representing its expulsion or delivery. The mean effective 
pressure of Fig. 8 is, as we have seen, 15.8, and that of Fig. 
9 is 16.4 lbs. But it must be remembered that the diameters 
of the two cylinders are quite different, and 16.4 lbs. in the 
I if" cylinder is only equal in power to 5.65 lbs. in the 20" 
cylinder, and 15.8 + 5.65 = 21.45 1^^^, a mean effective pres- 
sure quite close to what might have been expected for the 
entire operation of compressing air to 35 lbs. without any 
device for cooling the air. When we remember that the use 
of two cylinders instead of one for the same operation of com- 
pression means necessarily a greater first cost for the appar- 
atus, to the builder if not to the purchaser, a larger number 
of parts, increasing the liability to accidents and delays, and a 
greater amount of friction, both in the air and in the machine, 
to be constantly overcome, it is evident that two-stage com- 
pression of itself costs more than single-stage compression. 

While these diagrams were being taken the compressor 



TWO-STAGE AIR-COMPRESSION. 77 

was run at about 80 revolutions per min., or 240 feet of 
piston travel per min., throughout. At this speed the indi- 
cated horse-power of Fig. 8 for the first cylinder is 18.05. 
and that of Fig. 9 from the second cylinder is 6.46, their 
sum being 24.51. Fig. 10 is from the smaller cylinder when 
compressing to 70 lbs. The M.E.P. of Fig. 10 being 43.4, 
and the indicated horse-power being 17. i, the I.H.-P. for 
Fig. 8 being, as before, 18.05, their sum is 35.15. When 
compressing and delivering air at 70 lbs., as indicated by 
Figs. 8 and 10, it will be noticed that the I.H.-P. of the 
two cylinders is nearly equal, and it would thus seem that 
the ratio of the cylinder capacities to each other was ap- 
proximately correct for that pressure. The relative diame- 
ters and areas of the two cylinders may have been deter- 
mined upon this assumption. An incomplete theory is 
more easily satisfied than one which takes cognizance of all 
the conditions. 

The arrangement of the tandem, single-acting, two-stage 
compressing cylinders is about as bad a one as could be 
devised for an air-compressor, and no possible change in 
the relative capacities of the two cylinders can make it 
right. The trouble in the case is that while the sum of the 
indicated horse-powers as computed from the actual en- 
closed areas of the two cards is correct as representing the 
total horse-power consumed in the operation, it does not 
correctly represent the actual distribution of the resistances 
as encountered in the opposite strokes of the engine. The 
back pressure in the second cylinder, which thus far has 
not been thought of, imperatively demands recognition and 
accounting with as modifying the total resistances encoun- 
tered. The back-pressure line, or, perhaps more correctly, 
the return-pressure line, cxub, as we have seen, starting at 



78 COMPRESSED AIR. 

c, represents for nearly one-half the stroke the re-expansion 
of the contents of the air-passage. This re-expansion goes 
on in the passage and in the smaller cylinder combined until 
the point x is reached, when the compression going on in the 
larger cylinder has brought its contents up to the same pres- 
sure. Then after a short interval, xti, occupied in securing a 
sufficient excess of pressure, and in reversing the movement 
from expansion to compression, the compression continues 
from u to the end of the stroke, when the pressure of 35 lbs. 
is again reached. As the whole of Fig. 8 is always the same, 
no matter what may be the working pressure of the compres- 
sor, so that it is not below 35 lbs., so also the return line of 
the diagram from the second cylinder is always the same, and 
the only change in the pair of Figs. 8 and 9 or 8 and 10 for 
different delivery-pressures is in the upper line ba, the com- 
pression- and delivery-line of the second cylinder. AVhen 
compressing to 35 lbs. only there is no compression in the 
second cylinder, and its whole stroke is occupied in delivery. 
At the beginning of the stroke the resistance against the high- 
pressure piston is represented by the height of the vertical 
line bd. The resistance at any point of the stroke would 
be represented by a vertical line at that point drawn from 
the line ba down to the atmosphere-line, and the total 
resistance for the working-stroke is represented by the 
enclosed area, bdea. This means that the total back 
pressure, bdec, is to be added to, or, rather, is not to be 
deducted from, the work of the compression and delivery- 
stroke of the high-pressure cylinder. During this working- 
stroke of the high-pressure cylinder the low-pressure piston 
is making its return stroke and allowing its cylinder to refill 
with air at atmospheric pressure. The pressure upon each 
side of the low-pressure piston upon its return stroke is 



TWO-STAGE A IE-COMPRESSION. 79 

practically that of the atmosphere, and therefore no resist- 
ance of any magnitude is to be taken into account as in- 
creasing or diminishing the total work of the high-pressure 
cylinder for its delivery-stroke. When, however, the low- 
pressure cylinder is doing its work of compression, it is 
assisted in its work by the return or back pressure of the 
high-pressure cylinder, which acts upon the high-pressure 
piston in the same linear direction as the low-pressure 
piston is travelling. The back pressure, bdec^ which is 
added to the work of the high-pressure cylinder for its de- 
livery-stroke, as represented by the enclosed area bac^ is to 
be deducted from the work of the low-pressure cylinder for 
its compression-stroke as represented by Fig. 8. 

If now we go over the series of indicator-cards, comput- 
ing the indicated horse-power of each, adding the I.H.-P. of 
the back pressure to the I.H.-P. of each of the high-pressure 
cards, and deducting the same from the I.H.-P. of the low- 
pressure card, as above described, we find that the net re- 
sistance for the alternate strokes is very inequitably dis- 
tributed. The figures for compressing to 120 lbs. are also 
given to aid the comparison, although the delivery or high- 
pressure card for that pressure is not shown. The case will 
stand like this: 

M.E.P. of low-pressure cylinder 15.8 lbs., I.H.-P. 18.05. 
M.E.P. of return stroke of high pressure cylinder 20.1, I.H.-P. 7.88. 
Then 18.05 — 7.88=10.17, the constant net I.H.-P. for the 
compression-stroke of the low-pressure cylinder or the 
return stroke of the high-pressure cylinder. 

M.E.P. of high-pressure cyl. at 35 lbs. 16.4, I.H.-P. 6.46. 

•' *' " •* *' 120 'r 65.7 '' 25.89. 

Then adding to these results the I.H.-P. for the return 



80 COMPRESSED AIR. 

stroke, which should not have been deducted from the 
delivery-stroke, we have: 

6.46+ 7.88 = 14.34 when delivering at 35 lbs. 
17.1 +7.88:= 24.98 " " " 70 " 

25.89 + 7.88-33.77 " " " 120 " 

As these several results for the delivery-stroke are suc- 
cessively to be compared with the constant I.H.-P. 10.17 
for the initial compression-stroke, it will be seen that even 
when delivering the air at but 35 lbs. the delivery-stroke of 
the high-pressure cylinder takes nearly i^ times the power 
required for the return stroke. When compressing to 70 
lbs. under the above arrangement the delivery-stroke takes 
nearly 2\ times the power of the return stroke, and when 
compressing to 120 lbs. it takes more than 3 times as much. 

The total power required for the above compressor at 
the speed given is : 

35 lbs. — 10.17 + 14.34 = 24.51 

70 '' - 10.17 + 24.98 = 35.15 

120 " —10.17 + 33.77 = 43.94 

The volume of free air compressed and delivered at either 
pressure is 262 cu. ft. per min. 

The loss by friction in a two-stage compressor should be 
greater than in a single-stage compressor of the same free 
air capacity and working to the same pressure, and the 
total friction of single-acting cylinders must be propor- 
tionately greater than that of double-acting cylinders, 
so that if for a common single-stage double-acting com- 
pressor we allow 10 per cent for the total friction of the 
machine, it is probable that 15 per cent is not too great 
to allow for the arrangement that we have been considering 
above. 



CHAPTER IX. 

TWO-STAGE COMPRESSION, SINGLE-ACTING TANDEM, 
DOUBLE-ACTING TANDEM, AND CROSS-COMPOUND. 

I REFER again in this chapter to indicator-cards Figs. 8 
and lo from the single-actings two-stage, tandem air-cylin- 
ders delivering the air at 70 lbs. I reproduce these cards 
with some combinations resulting from them to show 
graphically how the net resistances are distributed through- 
out the alternate strokes. 

When the compressor is in operation, both pistons are 
always exposed to the atmospheric pressure upon the sides 
nearest to each other. The other side, or the compressing 
side, of the larger piston is also exposed to the atmospheric 
pressure, or very nearly so, during its intake stroke. The 
compressing side of the smaller piston is never exposed to 
the atmospheric pressure when the compressor is in opera- 
tion. During the intake stroke of the smaller cylinder, 
while it is receiving the air that is being compressed in the 
larger cylinder, its piston is subject to the pressure that is 
due to that initial compression. As both of the pistons 
are upon one rod, whatever pressure there may be upon the 
smaller piston when the larger piston is doing its work is 
just so much help for the larger piston, and consequently 
cbde of Fig. 10 is to be deducted from the total w^ork of 
Fig. 8. In Fig. n the area cbde^ representing this reacting 
pressure, has been reduced to the scale corresponding to 
the relative area of the larger cylinder, and has been super- 

81 



82 



COMPRESSED AIR, 



imposed upon Fig. 8. It will be seen that until the point / 
is reached the steam-cylinder, or whatever motor is em- 




mg, 11 

ployed, has "less than nothing " to do, and if the com- 
pressor were running slowly, it would be apt to give a per- 
ceptible jump ahead just after passing this centre. This 
has been actually observed to occur in a compressor of this 
type. In Fig. 12 the two diagrams have been combined 




Fig, 12 

into a single figure, with AB as the line of no resistance. 
This, it will be remembered, represents the distribution of 
the resistance for the compression-stroke of the larger pis- 
ton. For nearly one quarter of the stroke, considering here 
the air-cylinders only, and with no reference to the driving 
power of the steam-cylinders, the larger piston has a force 
behind it greater than the resistance in front of it. 
From the point / the net resistance begins to rise before 
the larger piston, and continues to rise until the ex- 
treme end of the stroke, except for a slight interval at 
the middle. Fig. 13 represents the resistance for the return 




Fig. 13 



Stroke, which is the delivery-stroke of the smaller piston. 
This diagram is the same as baed oi Fig. 10, but drawn to 
the scale of the larger cylinder for comparison. It has 



TWO-STAGE COMPRESSION, ETC. 83 

also for convenience been reversed. It is easy enough 
by a glance at Figs. 12 and 13 to see the difference in the 
resistances for the alternate strokes. If the compressor 
were delivering the air at 35 lbs., instead of at 70 lbs,, 
the upper line of Fig. 13 would approximately follow the 
dotted line ba, and the resistance would be practically uni- 
form for the entire stroke. Fig. 12, representing the alter- 
nate stroke, would remain precisely the same whether the 
smaller cylinder were delivering the air at 35 lbs., at 70 lbs., 
or at any higher pressure, and even at the lower pressure 
the resistance for this stroke would not be as great as for 
the delivery-stroke. 

It is evident that the resistance for the alternate strokes 
could not be equalized by changing the relative capacities 
of the two cylinders. To decrease the smaller cylinder 
would indeed tend toward an equalization of the resistances 
by allowing the first cylinder to do more work and com- 
press the air to a higher pressure ; but to raise the pressure 
in the first cylinder would be to defeat the purpose for 
which the two-stage compression is adopted — that of allow- 
ing a cooling of the air and a reduction of its volume before 
its compression is too far advanced. 

As Figs. 12 and 13 represent the resistances for the alter- 
nate strokes of single-acting cylinders, these resistances may 
be added together and w^e may combine them, as is done 




Fig. 14 

in Fig. 14, and we then have the diagram for either stroke 
of tandem double-acting cylinders of the same sizes. This 



84 COMPRESSED AIR. 

of course represents double the free air capacity of the 
single-acting cylinders. Fig. 15 is a theoretical diagram of 
a double-acting single-stage compression cylinder of the 
same capacity, the assumed compression-line being the 
mean of the adiabatic and the isothermal curves. The 
maximum resistance for the stroke in the two-stage double- 
acting compressor is only three fourths of the maximum 
resistance for the single-stage compressor. The resistance 
at the beginning of the stroke is not as low in the former as 




:Fig. 15 



in the latter, and the distribution of the resistance over the 
whole stroke is decidedly more uniform, x^s to the total 
effective resistance for the stroke, as we have here devel- 
oped it, the two-stage, compressor shows no advantage over 
the single-stage even while ignoring the additional friction 
of the former. In fact, the mean effective resistance of 
Fig. 15 is somewhat less than that of Fig. 14. This might 
have been expected, because in the cylinders from which 
Fig. 14 was evolved the full benefits of water-jacketing were 
not employed, the cylinder-heads, for instance, not being 
jacketed. 

We know tolerably well the importance of employing all 
available means (if they don't cost too much) of cooling the 
air while it is undergoing compression; and as the two-stage 
method of compression is only adopted for the sake of the 
cooling that may be effected between the stages, it may be 



TWO- STAGE COMPRESSION, ETC. 85 

well right here to look a little into the operation of a cooler, 
or, as it is commonly called, an *' intercooler," placed be- 
tween the cylinders of a tandem two-stage air-compressor. 
It is assumed and asserted that by the use of the inter- 
cooler a complete cooling of the air, and of all the air, 
compressed by the first cylinder is effected before it is sub- 
jected to the second and final compression and delivery. 
Indicator-cards would show conclusively, by the relative 
volume delivered to the second cylinder, the actual cooling 
that was accomplished. I regret that I am not now able to 
present indicator-cards from a compressor of this type. I 
cannot learn that any actual cards from an American 
double-acting, tandem, two-stage compressor with an inter- 
cooler have ever been published. 

At the beginning of the operation of compression in 
a compressor of this type, remembering, as I have remarked 
before, that the function of the first cylinder is entirely one 
of compression, and that, if either cylinder is to be called 
distinctively the " compressing " cylinder, it should be the 
first one rather than the second, the body of air to be acted 
upon by the first piston consists, at the beginning of any 
stroke, of the entire contents of the first cylinder and also 
of the air contained in the intercooler at the time and in 
the passages connecting the intercooler with each cylinder. 
As the compressing piston advances in the first cylinder the 
total compression-chamber at any time after the beginning 
of the stroke consists at that time of the remaining portion 
of the first cylinder still untraversed by its piston, of the 
intercooler and its connecting passages as before, and 
of that portion of the second cylinder that has been 
vacated by its retreating piston. The actual situation is 
not quite as simple as our statement of it here, as we can 



86 COMPRESSED AIR. 

see a little later. In standard compressors of the type that 
we are considering the piston areas and consequently the 
cubical capacities of the cylinders usually bear to each other 
about the ratio of lo : 4. Now representing the capacity 
of the first cylinder by 10, that of the passage connecting 
it with the cooler by 2, of the cooler itself by 2, of the 
passage to the second cylinder by 2, and the total capacity 
of the second cylinder by 4, we may be able to see what the 
intercooler has to operate upon at any given time, and what 
chance it has to completely cool all the air. I assume, of 
course, that the cooler does thoroughly cool all the air that 
passes through it, and at the pressure at which it passes 
through. I see no reason why it should not be made effi- 
cient in this respect. 

The operation at successive stages of the compression- 
stroke is as follows : At the beginning of the stroke of the 
first cylinder the entire body of air to be compressed is 
represented by 16, comprised like this : The contents of 
the first cylinder 10, passage to cooler 2, contents of cooler 
2, passage to second cylinder 2. Of this volume of air only 
the first 10 parts, the contents of the first cylinder, is ** free 
air " The remainder, the contents oi the cooler and the 
connecting passages, having been compressed upon the 
previous stroke to the pressure at which the air is finally 
delivered to the second cylinder, and at the end of the 
stroke having been shut off by itself apart from either cylin- 
der, stands now at a pressure somewhat above 35 lbs. gauge. 
As the stroke goes on, and the piston of the second cylin- 
der recedes, this air in the cooler and passages begins to 
re-expand, and to flow into the second cylinder, and the 
pressure of this air consequently falls. At the same time 
compression is going on without cooling in the first cylin- 



TWO-STAGE COMPRESSION, ETC. §7 

der. The total free air contents of the first cylinder are 
compressed independently until the middle of the stroke is 
reached, or a little beyond that, and a pressure of about 20 
lbs. is attained in the cylinder without any of the cooling 
and power-saving effects of the intercooler being felt upon it. 
Practically none of the air of any compression-stroke flows 
through the intercooler until after the middle of that stroke 
is reached. Assuming that the pressures in the compress- 
ing cylinder and in the cooler and passages have become 
equal when the middle of the stroke is reached, and that at 
that point the piston of the first cylinder begins to act upon 
the whole body of air at once, the air then under compres- 
sion will be : Contents of first cylinder 5, of passage to 
cooler 2, of cooler 2. of passage to second cylinder 2, and 
contents of second cylinder 2 — total 13 ; and -^ of this — 
.307 has already passed the cooler and can be no more 
cooled by it, and y\ = .538 has not yet reached the cooler, 
and has been compressed thus far without any cooling 
effect w^hatever from it. At three quarter stroke the body 
of air under compression will be distributed as follows : 
Remaining contents of first cylinder 2.5, passage to cooler 2, 
cooler 2, passage to second cylinder 2, and contents of sec- 
ond cylinder 3 — total 11.5; and of this body s/11.5 = .434 
has already passed the cooler and cannot be further af- 
fected by it, and 4.5/1 1.5 = .39 has not yet reached the 
cooler, and has not been cooled at all by it. When the end 
of the stroke is reached, the air is distributed like this : 
First cylinder o, passage 2^ cooler 2, passage 2, second cylin- 
der 4 — total 10 ; and of this -f-^ = .2 has not yet reached 
the cooler and has undergone the whole compression from 
atmospheric pressure without cooling, and all of the con- 



88 



COMPRESSED AIR, 



tents of the second cylinder have been compressed and 
heated more or less after passing the cooler. 

The intercooler applied in this way would seem to be a 
rather crude and not very efficient device and when con- 
fidence in the virtues of the intercooler leads to the discard 
ing of the most valuable feature of water-jacketing, — the 
jacketing of the cylinder-heads, — and when, for the same 
work of compression, two cylinders are employed instead of 
one, with the consequent increase of friction in the ma- 
chine, and with the increased friction also of the air past a 
double set of valves and through longer and more tortuous 
passages, it would surely seem to require a voluminous 
argument to show in the system any superiority over the 
single-cylinder completely water-jacketed compressor for 
the commonly employed working pressures. 

Figs. 1 6 and 17 are indicator-cards from two-stage air- 
cylinders operated by cross-connected Corliss engines with 




Fig, le 



the cranks at right angles. The piston rod of each steam- 
cylinder in this style of compressor is carried back through 
the head and into the air-cylinder, the low-pressure, or 
intake, air-cylinder being placed tandem to one steam-cylin- 
der, and the high-pressure, or delivery, air-cylinder being 
connected in the same way to the other steam-cylinder. 



TWO-STAGE compression; ETC. 



89 



These cards are reproduced here to show the characteristics 
of this style of compressor as compared with the tandem 
air-cylinder arrangement. They may to the general reader 
possess an additional interest from the fact that the original 
cards were taken in South Africa, where there are now in- 
stalled a large number of high-duty air-compressors of 
American manufacture. As the cards have been twice 




Fig, rt 

retraced, they should not be too closely scrutinized. The 
intake cylinder was 31" dia. X 42"stroke, and the delivery- 
cylinder 19.5" dia. X 42" stroke. The cards were taken 
with the compressor running at 40 revolutions per minute. 
The scale of the first card is 20, and that of the second 
card is 60. 



CHAPTER X. 
THE POWER COST OF COMPRESSED AIR. 

What is the actual power cost of a cubic foot of com- 
pressed air at any given pressure ? This is only one end of 
the question of economy in employing compressed air for 
power transmission, and besides the ends of it there is a 
middle of some magnitude. The question of practical 
economy has many complications, and whether air shall be 
employed in a given case may be determined by considera- 
tions far removed from those that we would recognize as 
bearing upon the economy of it. There are many cases 
where at the present time the use of compressed air is im- 
perative, whatever its cost ; but still as the bill has to be paid 
it is well to compute it. In considering the actual cost of 
compression we will not now look into all the possible 
economies of the case, but will try to get at the actual cost 
according to the common practice of air-compression at 
the present time. 

Say, then, that we have a steam-actuated air-compressor, 
with steam- and air-cylinders both 20" dia. X 24" stroke, at 
75 revolutions per min., using steam at 80 lbs. and com^ 
pressing air to 80 lbs. The case will then be like this : 

Power required by air-cylinder : 

20^^ X .7854 X s^.6 X 300 -^ 33,000 = 104.53 H.-P. 
I04-53 + 10 per cent. = 114.98 H.-P. 

90 



THE POWER COST OF COMPRESSED AIR, 9I 
Volume of free air compressed by air-cylinder : 

20' X .7854 X 300 -^ 144 = 654.5. 

654.5 — 10 per cent = 589 cu. ft. free air. 
589 X .1552 — 91.4 cu. ft. at 80 lbs. 

Power of steam-cylinder (steam 80 lbs., cut-off .25, M.E.P. 
40.29): 

20^ X .7854 X 40.29 X 300 -^ 33,000 = 115.06 H.-P. 

Volume of steam used : 

20' X .7854 X 75 ~ 144 = 163.62. 
163.62 -|- 10 per cent = 180 cu. ft. 

Here 180 cu. ft. of steam at 80 lbs. produce 94 cu. ft. of 
air at 80 lbs., or i cu. ft. of air at this pressure costs 
nearly 2 cu. ft. of steam. It should be remembered that 
the same ratio will not necessarily hold good for other 
pressures. For lower air pressures the steam will have a 
little more advantage, and for higher pressures it will have 
a little less. The mean effective resistance assumed for 
the air-cylinder is the theoretical resistance with no cool- 
ing of the air. In practice the actual resistance is some- 
what less than this, but the difference between the air- and 
the steam-cards, or the friction loss of the machine, is also 
usually more than 10 per cent, so that few of the common 
compressors in use will at their best give any better results 
than the above. 

The following table, V, gives the horse-power required to 
compress one cubic foot of free air per minute to a given 
pressure, also the horse-power required to furnish a cubic 
foot of air at the given pressure ; or, in other words, the 
power cost of the operation of air-compression is exhibited 



92 



COMPRESSED AIR. 



Table V. 

TABLE SHOWING THE HORSE-POWER REQUIRED TO COMPRESS I 
CUBIC FOOT OF FREE AIR PER. MINUTE TO VARIOUS GAUSE 
PRESSURES, ALSO THE POWER REQUIRED TO DELIVER I CUBIC 
FOOT OF AIR AT THE GIVEN PRESSURE. 





Compressing i Cu. Ft. of 


Delivering 1 C 


u. Ft. per Min. 




Free Air per Min. to 


of Air Compressed to the 


I 


given Pressure. 


Pressure given. 


Gauge 










Pressure. 


2 


3 


4 


5 




Compression at 


Compression 


Compression at 


Compression 




Constant 


without 


Constant 


writhout 




Temperature. 


Cooling. 


Temperature. 


Cooling. 


5 


.01876 


.01963 


.02514 


.0263 


lO 


.03325 


.03609 


.05586 


.06399 


15 


.04507 


.05022 


.09105 


.10145 


20 


.05506 


.06283 


.12994 


.14829 


25 


.06366 


.07422 


.17191 


.20043 


30 


.0713 


.08464 


.21678 


.25734 


35 


.0782 


.09425 


. 26445 


.31872 


40 


.084305 


. IQ324 


.31375 


.38422 


45 


.08954 


.11166 


.36368 


•45353 


50 


.09508 


.11952 


.41848 


.52605 


55 


.09936 


.12702 


.47112 


.60227 


60 


. 10402 


.13418 


.52855 


.68181 


65 


. 10808 


. 14028 


.58612 


. 76079 


70 


.11245 


.14718 


.64812 


.8483 


75 


.11629 


.15373 


.70952 


.93795 


80 


.11926 


.15971 


.76843 


I . 02906 


85 


.1224 


.16555 


.83039 


1.1231 


90 


.12558 


. 17096 


.89444 


I. 2176 


95 


.12886 


.17629 


.96164 


I. 3148 


100 


.13121 


.18153 


1.0243 


1.4171 



both from the beginning and from the termination of it. 
From either standpoint the power required is given both 
for isothermal and for adiabatic compression, in the one 
case assuming that the air remains at its initial tempera- 
ture during the compression, and in the other case that the 
air as heated by the compression is not cooled during the 
operation. The power required as given in the table is the 



THE POWER COST OF COMPRESSED AIR. 93 

theoretical power, and no allowance is made for the inevi- 
table losses of power that occur in its actual application, 
and of course it makes no difference what may be the 
source of the power, or the economy with which it may be 
developed or applied. The power employed may be 
steam, with or without cut-off or condensation, water-power, 
electricity, manual power, or anything else. When the vol- 
ume of free air required to be compressed per minute is 
known, or the volume of air at the given pressure required 
to be furnished, the theoretical power required may be 
found by multiplying the number of cubic feet required by 
the power required for i foot, as here given. In the last 
column of the table although the compression is assumed 
to be adiabatic the air is supposed after delivery to have 
cooled to normal temperature, and to have assumed its 
practically available volume, and the i cu. ft. of com- 
pressed air represented in column 5 is precisely the same as 
the I cu. ft. in column 4. 

In the use of this table the second column, showing the 
power cost of isothermally compressing i cu. ft. of free air 
to the given pressure, represents the ideal and unattainable, 
but still the only rational and natural, standard of efficiency 
in air-compression. Whatever the actual power employed 
may exceed the values in this column is the irrecoverable 
cost of compression. In comparing the performance of a 
steam-actuated air-compressor with this standard we shall 
find at least four different sources of loss in the operation 
of compression, and all requiring some deduction from the 
ideal efficiency. Few persons in dealing with compressed 
air recognize and make the necessary allowances and de- 
ductions for all of these sources of loss, and in consequence 
the efficiencies of the air-compressors of the day are gener- 



94 COMPRESSED AIR. 

ally represented to be much higher than they actually are. 
In deploring the low ultimate efficiencies in compressed-air 
systems we may still find great losses in the compression 
end of them, notwithstanding all the boasted " modern im- 
provements." 

The first deduction to be made is for the friction of the 
machine, and is accurately represented by the difference in 
the mean effective pressures in the air, and in the steam- 
cylinders, assuming the areas and strokes of the two cylin- 
ders to be the same. This difference is often found to be 
surprisingly low. In some large Corliss compressors, where 
the air-cylinders are placed tandem to the steam-cylinders, 
the piston-rod from the steam-cylinder being continued 
into the air-cylinder to operate its piston, the total loss of 
power in the friction of the engine often ranges as low as 
5 per cent, where the friction of the same steam-engine if 
transmitting all of its power through its crank-shaft would 
exceed jo per cent. Compressed air evidently here has a 
great advantage over electricity, and the first power loss in 
an electric system^ in driving the generator by means of a 
steam-engine, and including the friction of the generator, is 
necessarily from two to three times as great as the loss in 
operating ihe air-cylinder of a steam-actuated compressor 
of the best type. The friction loss in the common straight- 
line, direct-acting air-compressors may generally be as- 
sumed at lo per ceni, and is seldom found lower than that. 
Some statements of air-compressor efficiencies are made 
upon the friction loss alone, and in the last-mentioned in- 
stance the efficiency of the compressor would be stated as 
90 per cent, with no hint of any other losses, which is 
absurd. 

The second source of loss to be reckoned with is in the 



THE POWER COST OF COMPRESSED AIR, 95 

increase of temperature and reduction of weight of air ad- 
mitted to the cylinder for compression. This loss is sel- 
dom recognized, and still more rarely made the subject of 
actual computation. It is difficult to determine it accu- 
rately, because it is the one detail in the cycle of operations 
in air-compression about which the indicator-diagram has 
nothing to say. It is evident, however, that there must be 
some loss from this source in almost every case. As the 
air is always heated by compression, and at best only par- 
tially cooled, the cylinder is heated by it, and after continu- 
ous compression becomes quite hot. Water-jacketing only 
partially cools the inner surfaces of the cylinder, and some 
parts of it and the heads and usually all of the piston are 
not cooled at all by the water. The air, which when 
heated we find to give up its heat so quickly in transmis- 
sion, is also heated with equal celerity when the conditions 
are reversed, and it cannot pass Jthrough heated passages 
into a heated chamber, which the cylinder is, without being 
heated and increased in volume, so that a less weight or 
actual quantity of air is sufficient to fill the cylinder. The 
loss in many cases from this source is perhaps light, but in 
some cases there can be little doubt that it exceeds the 
friction loss of the compressor. If air whose normal tem- 
perature is 60° is actually at 120° at the moment when 
compression begins in the cylinder, the weight of air pres- 
ent is less than 90 per cent of the same volume at its orig- 
inal temperature. 

The third loss of power in air-compression is due to the 
heating of the air during the compression, and to the 
greater force required for the compression on account of 
this heating. This is the one source of loss that is gener- 
ally recognized, and too often treated of as the only one. 



96 COMPRESSED AIR, 

The loss in this case is represented by the percentage of 
excess of mean effective pressure above that required for 
isothermal compression. In compressing to 70 lbs. the 
M.E.P. for isothermal compression is 26, and for adiabatic 
compression it is 33.73, and the mean of the two is 29.87. 
The excess of the adiabatic above the isothermal is 29.7 per 
cent, and the excess of the mean above the isothermal is 
still 14.85, or say 15 per cent. No compressor within my 
knowledge does its compression to 70 lbs. with less than 15 
per cent of loss except by devices that increase the friction 
of the machine or add to the power required or to the cost 
of operation in some way. 

The fourth source of power loss in air-compression lies 
in the fact that while the indicator-cards show, as they do, 
that the M.E.P. tor the compression-stroke is above the 
mean of the isothermal and the adiabatic pressures, or 
when compressing to 70 lbs. more than 15 per cent above 
isothermal compression, the volume of free air compressed 
is never a cylinderful. The figures in the formulas and in 
the tables are based upon the assumption that a certain 
volume of air is compressed, and when applied to the cyl- 
inder of a compressor, the actual capacity of the cylinder, 
or the net area multiplied by the stroke, is the volume rep- 
resented. It is of course the fact that the volume actually 
compressed is always somewhat less than this. There is a 
loss at each end of the stroke. Compression of the air at 
full atmospheric pressure does not begin precisely at the 
beginning of the stroke, and all of the air is not expelled 
by the piston at the end of the stroke. It is custom- 
ary with compressor-people to say that clearance in the 
air-cylinder at the end of the stroke does not mean loss of 
power, but only loss of capacity, because the power which 



THE POWER COST OF COMPRESSED AIR. 9/ 

has been expended in the compression of the air filling the 
clearance-space is returned to the piston by the re-expan- 
sion of the air when the piston makes its return stroke. 
The clearance does, however, practically represent an actual 
loss of power, or an expenditure of power without any result, 
because the evidence which the clearance gives is so gener- 
ally ignored, and every stroke of the piston is assumed to 
compress and deliver free air to the full capacity of the 
cylinder, which it certainly never does. 

In practice these four items of loss of power in compres- 
sion occur in different combinations, such as lo, lo, 17, 
10 = 60.5 per cent net efficiency or 7, 2, 15, 5 = 73.6 per 
cent net efficiency. It is safe to say that the ultimate effi- 
ciency never goes as high as 80 per cent, while it often goes 
below 60 per cent. If any air-compressor builder feels 
aggrieved over this statement, a fine opportunity is opened 
for a demonstration of a higher efficiency. Indicator-cards 
from air- and steam-cylinders are full and conclusive evi- 
dence as to three of the four items of loss enumerated 
above, and it might be profitable to make an exhibit of 
these, and if it proved to be creditable, we could be gener- 
ous in our estimates of the one concerning which no proof 
seems to be easily procurable. 



CHAPTER XL 
THE POWER VALUE OF COMPRESSED AIR. 

Those of us who are not wise enough to consider well 
before buying it what a thing will be worth to us are very 
apt to be looking it over anxiously after the purchase to 
see what sort of a bargain we have got. As in the last 
chapter we learned the approximate power cost of a cubic 
foot of compressed air at a given pressure, we now naturally 
want to know what it is worth to us. We realize that in 
the compression it is costly, if, indeed, we do not think that 
it costs too much, and yet we go on using it more and more, 
and find profit in doing so. Our bargain is really worse 
than appears thus far ; for if we take our compressed air 
and go to use it as we use steam, or if we substitute it in a 
place where we have been using steam, as in a steam-engine, 
we soon find that a cubic foot of air at any given pressure 
is not worth as much, in power, as a cubic foot of steam at 
the same pressure. 

The accompanying diagram, Fig. i8, shows how this can 
be so. Here we have i volume of steam and the same 
of air, both at loo lbs. gauge pressure, and each success- 
ively expanded through several additional volumes until 
the pressure of each falls below that of i atmosphere. 
It is readily seen that the two expansion-lines are very dif- 
ferent, and that the mean effective pressure of the steam is 
decidedly higher than that of the air. Thus i volume of 

98 



THE POWER VALUE OF COMPRESSED AIR. 99 

Steam at loo lbs. gauge, represented by the length of the 
line A\^ reaches atmospheric pressure after expansion to 
about six and a half times the original volume, while the 
same volume of air drops to the same pressure after expan- 



fej 1 


^-^^^^^^^ 


^^-^^^ 


^^z^^ 


^^\^^ 


y y 


/ / 


/ / 


/ / 


/ / 


r / 


t^ 1 


I k k 


1 ^ 


i h 


/ / 


/ / 


/ ' 




/ 


/ 


/ 


t ji 




















! 1 



s § 



sion to a little over four times its original volume. The 
mean effective pressure for the steam, taking the whole ex- 
tent of the diagram, or cutting off at i stroke, is 27.38 lbs., 
while the M.E.P. for air under the same conditions is 19.51 



ICX> COMPRESSED AIR, 

lbs., or only 71 per cent of the former. As with this cut- 
off the terminal pressures are below the atmosphere, the 
entire mean effective pressures are not properly " effective *' 
or available or comparable. At \ cut-off the M.E.P. for 
steam is 51.93, and for air it is 44.19, or 85 per cent, which 
looks a little better for the air, but in this case the terminal 
pressure of the steam is 11 lbs. gauge, and some of its 
power is lost through the exhaust. 

This diagram is equally applicable for any other initial 
pressure below 100, by taking as the measure of volume 
the length of a horizontal line drawn from the line AB to 
the expansion-line at the given pressure, and taking each 
repetition of this length horizontally as representing an 
additional volume. Thus at 60 lbs. pressure i volume of 
steam is represented by i^, and 2 volumes would be rep- 
resented by 3, and at the intersection of the vertical line 
marked 3 we find that the steam pressure has fallen to 21 
lbs., which is nearly correct. One volume of air at 60 lbs. 
is represented by about if of the diagram-spacing, and 2 
volumes would consequently be 2% of the spaces, and here 
we find the air pressure to be 13 +> which is the correct 
terminal pressure for air at 60 lbs. cut-off at \ stroke, or 
expanded to double the volume. We may take any sec- 
tion of this diagram as representing, theoretically, an indi- 
cator-card either for steam or air, but we cannot take both 
the steam- and the air-cards and compare them by placing 
one upon the other, because the lengths of the two cards 
will not coincide. 

Fig. 19 is a theoretical card, scale 40, showing both 
steam and air expanded to atmospheric pressure at the end 
of the stroke. In this case the air-line is outside of and 
above the steam-line, and, of course, represents a higher 



THE POWER VALUE OF COMPRESSED AIR, lOI 

mean effective power, but it is at the expense of a much 
larger initial volume. The M.E.P. for air filling a cylinder at 
an initial pressure of loo lbs. for a sufficient portion of the 




stroke and then expanding (without loss or gain of heat) so 
that it reaches atmospheric pressure at the end of the 
stroke will be 41.6 lbs. The M.E.P. for steam under the same 
conditions will be 32.46. The volume of air used will be 



102 COMPRESSED AIR, 

•2353, while the volume of steam, will be .1471. If the air 
gave the same M.E.P. in proportion to its volume, it would 
be .1471 : 2353 : : 32.46 : 51.9, instead of 41.6, and the greater 
comparative efficiency of steam under the conditions is 
41.6 : 51.9 : : I : 1.247, or nearly 25 per cent. 

As the expansion of the air here exhibited is adiabatic, 
its temperature, at least for the latter portion of the expan- 
sion, would be below that of the cylinder containing it, and 
the air would be heated and expanded, rather than cooled, 
by its surroundings ; so that there need be no apprehension 
that the expansion-line would be below the theoretical, or 
that there might be still some lurking losses to arise and 
confront us. The essential difference in an engine or 
motor to be driven by compressed air instead of steam is a 
later cut-off for the same initial pressure. This later cut- 
off develops the paradox that although air has less available 
power than steam, volume for volume, the same cylinder with 
the same pressure will develop more power with air than 
with steam, both being used at the point of highest efficiency. 

I offer herewith a table, VI, showing the mean effective 
and terminal pressures for both steam and air at various 
points of cut-off and for different gauge pressures from 50 to 
100. Gauge pressures are given throughout except when 
below atmosphere when the absolute pressures are given in 
italics. It is thought that in this way the table will be more 
serviceable to the general mechanic than if the absolute 
pressures were given throughout. Nothing is said of the 
initial temperature of the air, as that would not affect the 
rate of expansion or the mean effective pressure. 



THE POWER VALUE OF COMPRESSED AIR, 10% 

Table VI. 

TABLE OF MEAN EFFECTIVE AND TERMINAL PRESSURES OF STEAM AND 
AIR AT VARIOUS POINTS OF CUT-OFF AND FOR DIFFERENT GAUGE- 
PRESSURES FROM 50 TO 100 LBS. 

All pressures given in the table are gauge pressures, except where they fall 
below atmosphere, when the absolute pressures are given and printed in full face. 





INITIAL PRESSURE 50 


LBS. 




Point 

of 

Cut-off. 


Mean Steam 
Pressure. 


Mean Air 
Pressure. 


Terminal 

Steam 
Pressure. 


Terminal 

Air 
Pressure. 


.05 


12.12 


8.87 


2.69 


•95 


iV 


1439 


10.8 


3-41 


1.31 


.10 


5.44 


1.2 


5.63 


2.54 


i 


8.95 


4.51 


7 13 


3.47 


.15 


10.18 


7.62 


8.65 


4.49 


A 


16.55 


11.96 


10.97 


6.14 


.20 


17.9 


13.84 


11.7s 


6.74 


.25 


22.83 


18.45 


14.9 


9 23 


.30 


27.11 


23.05 


3.08 


II 93 


4 


29.66 


25.84 


5.22 


13.83 


.35 


30.86 


27.17 


6.3 


14.82 


f 


32.56 


29.07 


7.92 


1.34 


.40 


34.15 


30.87 


9.55 


2.88 


.45 


37.03 


34.18 


12.84 


4. II 


.50 


39-54 


37.12 


16.12 


7.49 


.60 


43.61 


41.98 


22.77 


16.66 


f 


44.44 


42.99 


24.44 


18.53 


\ 


45.67 


44.52 


27.24 


21.73 


.70 


46.54 


45.6 


29.49 


24.33 


.75 


47.64 


46.98 


32.88 


28.34 


.80 


48.52 


48.08 


36.27 


32.47 


1 


49.43 


49.26 


41.4 


38.85 


.90 


49.64 


49.53 


43.11 


41.03 



104 



COMPRESSED AIR, 
Table W,— {Continued.) 



INITIAL PRESSURE 6o LBS. 



Point 

of 
Cut-off. 


Mean Steam 
Pressure. 


Mean Air 
Pressure. 


Terminal 

Steam 
Pressure. 


Terminal 

Air 
Pressure. 


.05 


13.99 


10.23 


3.1 


I.I 


tV 


1. 61 


12.46 


3.93 


1. 51 


.10 


8.58 


3.69 


6.49 


2.93 


\ 


12.64 


7.51 


8.22 


4.01 


.15 


16.37 


II. I 


9 99 


5.21 


A 


21.41 


16. II 


12.66 


7.08 


.20 


22.96 


17.7 


13.56 


7.77 


.25 


28.75 


23.6 


2.19 


10.65 


.30 


33.59 


28.9 


5.87 


13.77 


r * 


36.54 


32.13 


8.34 


.96 


.35 


37.92 


33.66 


9.58 


2.33 


/ « 


39.87 


35.85 


II. 8 


3.85 


.40 


41.71 


37.93 


13.22 


5.64 


.45 


45.03 


41.75 


17. 1 


10.71 


.50 


74.94 


45.14 


20.91 


13.26 


.60 


52.62 


50.75 


28.59 


21.53 


* 


53.58 


51.92 


30.51 


23.69 


* 


55.01 


53.67 


33.74 


27.94 


.70 


56.01 


54.93 


36.34 


30.39 


.75 


57.28 


56.52 


40.24 


35.01 


.80 


58.29 


57.79 


44.06 


39.78 


i 


59.34 


59.15 


50.07 


47.14 


.90 


59.58 


59.46 


52.05 


49-65 



THE POWER VALUE OF COMPRESSED AIR. lOj 
Table VI. — {Continued,) 



INITIAL PRESSURE 70 LBS. 



Point 

of 
Cut-off. 


Mean Steam 
Pressure. 


Mean Air 
Pressure. 


Terminal 

Steam 
Pressure. 


Terminal 

Air 
Pressure. 


.05 


1.06 


II. 6 


3.52 


1.23 


A 


3.82 


14.12 


4.46 


1. 71 


.10 


11.73 


6.19 


7.36 


3.32 


i 


16.33 


10.51 


9.32 


4.54 


.15 


20.55 


14.58 


11.32 


5.88 


A 


26.26 


20.25 


1437 


8.03 


.20 


28.02 


22.06 


.37 


8.81 


.25 


34.47 


28.74 


4.49 


12.07 


.30 


40.07 


34.75 


6.65 


.6 


* 


43.41 


38.41 


11.45 


3.09 


.35 


44-97 


40.15 


12.86 


4.38 


f 


47.19 


42.63 


14.98 


6.36 


.40 


49.27 


44.99 


17. 1 


8.39 


.45 


53.04 


49.31 


21.38 


12.61 


.50 


56.33 


53.16 


25.69 


17 


.60 


61.64 


59-51 


34.4 


26.4 


f 


62.73 


60.84 


36.58 


28.85 


1 


64.34 


62.83 


40.24 


33.03 


.70 


65.48 


64.25 


43.19 


36.44 


.75 


66.92 


66.05 


47.61 


41.68 


.80 


68.07 


67.5 


52.05 


47.08 


1 


69.26 


69.03 


58.75 


55.43 


.90 


69.53 


69.38 


60.99 


58.27 



io6 



COMPRESSED AlR, 
Table VI.— {Continued.) 



INITIAL PRESSURE 8o LBS. 



Point 

of 

Cut-off. 


Mean Steam 
Pressure. 


Mean Air 
Pressure. 


Terminal 

Steam 
Pressure. 


Terminal 

Air 
Pressure. 


.05 


2.72 


12.96 


3.93 


1.39 


tV 


6.04 


.78 


4.98 


1.92 


.10 


14.87 


8.68 


8.22 


3.71 


4 


20. 01 


13.51 


10.42 


5.08 


.15 


24.73 


18.06 


12.65 


6.57 


f^ 


31.12 


24.4 


1.04 


8.97 


.20 


33.08 


26.6 


2.18 


9.85 


.25 


40.29 


33.89 


6.78 


13.49 


.30 


46.55 


40.61 


11.43 


2.44 


\ 


50.28 


44.69 


14.56 


5.22 


.35 


52.03 


46.64 


16.14 


6.66 


1 


54.51 


49.41 


18.5 


7.88 


.40 


56.83 


52.05 


20.88 


II. 14 


.45 


61.04 


56.9 


25.66 


15.86 


.50 


64.72 


61.18 


30.48 


20.81 


.60 . 


70.76 


68.28 


40.21 


31.27 


f 


71.87 


69.76 


42.65 


34.01 


i 


73.68 


71.99 


46.74 


38.68 


.70 


74.95 


73.57 


50.03 


42.49 


.75 


76.56 


75.59 


54.97 


48.35 


.80 


77.84 


77.2 


59.94 


54.38 


f 


79.17 


78.92 


67.43 


63.81 


.90 


79.47 


79.31 


69.93 


66.89 



THE POWER VALUE OF COMPRESSED AIR, I07 
Table V\.— {Continued.) 



INITIAL PRESSURE 90 LBS. 



Point 

of 
Cut-off. 


Mean Steam 
Pressure. 


Mean Air 
Pressure. 


Terminal 

Steam 
Pressure. 


Terminal 

Air 
Pressure. 


.05 


4.59 


14.33 


4.34 


1.54 


-h 


8.25 


2.95 


5.51 


2.12 


.10 


1S.02 


II. 17 


9.09 


4.1 


i 


23.7 


16.52 


II. 51 


5.61 


.15 


28.92 


21.55 


13.98 


7.26 


A 


35.97 


28.55 


2.73 


9.92 


.20 


38.15 


30.78 


3.99 


10.88 


.25 


46.11 


39.04 


9.07 


14.91 


.30 


53.02 


46.46 


14.22 


4.27 


4 


57.17 


50.98 


17.67 


7-35 


.35 


59.08 


53.13 


19.42 


8.95 


1 


61.82 


56.2 


22.03 


11.39 


.40 


64.4 


59-11 


24.65 


13.88 


.45 


69.05 


64.45 


• 29.95 


19. II 


.50 


73.11 


69.19 


33.27 


24.56 


.60 


79.67 


77.05 


46.02 


36.14 


1 


81.02 


78.69 


48.72 


39.16 


t 


83.01 


81.14 


53.23 


44.33 


.70 


84.42 


82.9 


56.88 


48.54 


• 75 


86.19 


85.12 


62.34 


55.02 


.80 


87.61 


86.91 


67.83 


61.69 


1 


89.08 


88.81 


76.1 


72 


.90 


89.42 


89.24 


78.88 


75.52 



io8 



COMPRESSED AIR, 
Table VI. — {Continued.) 





INITIAL PRESSURE lOO 


LBS. 




Point 
of 

Cut-off. 


Mean Steam 
Pressure. 


Mean Air 
Pressure. 


Terminal 

Steam 
Pressure. 


Terminal 

Air 
Pressure. 


.05 


6.45 


.69 


4.76 


1.69 


A 


10.24 


4. II 


6.03 


2.32 


.10 


21.16 


13.66 


9.95 


4.49 


i 


27.38 


19.51 


12. 61 


6.15 


.15 


33.1 


25.03 


.31 


7.95 


A 


40.83 


32.71 


4.42 


10.89 


.20 


43.21 


35.14 


5.79 


11.92 


.25 


51.93 


44.19 


11.36 


1.33 


.30 


59.5 


53.32 


17 


6. II 


i 


64.02 


57.26 


20.78 


9.48 


.35 


66.14 


59.62 


22.69 


11.23 


f 


69.14 


62.98 


25.56 


13.89 


.40 


71.96 


66.16 


28.43 


16.64 


.45 


77.05 


72.02 


34.23 


22.36 


.50 


81.5 


77.21 


40.06 


28.33 


.60 


88.69 


85.82 


51.83 


41.01 


f 


90.15 


87.61 


54.79 


44.32 


# 


92.19 


90.32 


59.73 


49-97 


.70 


93.89 


92.22 


63.72 


54.59 


.75 


95.83 


94.66 


69.7 


6^^.69 


.80 


97.38 


96.61 


75.72 


68.99 


1 


98.99 


.,8.7 


84.78 


80.28 


.90 


99.36 


99.17 


87.82 


84.14 



THE POWER VALUE OF COMPRESSED AIR, IO9 



O CX3 0\<J\ ■*». 10 O 00 0\Kl\ 4»^ N O 00 ON(^ 



4>. to O O 00^ ONOl ^ W to M 






(OMMtOtOMMMWI-IHMMM 

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g ooo\o-^ w o ooo\04^ K) o ooo\0-^ » 
oooooooooooooooooo 



OJ OJ N> i-i 



2? 



o 2. 



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00 M vj 4k. M <>, On OOCn vO -^. vO OJ w OOU) ^ Ln N) >0 On-^ r* OOOi hJ 
to Cyi vj 



vO VO 00^ 0\tn +. 4>. U) W » 



VO ON-f>. 00 H VD Oi4». -OiVO-«-a4>. K3vOW»«4-P>- M OsO OJ <^ H 4». 00 10 On 
to vq oj O 00-fk vO -f»> 00 ONLa O. « Ui N OOOJ m 00 0\-^ h vO On-»>. 10 

to O O.OJ 



10 M o OOvj 0\tn Or 4^ to to M 



(OLa Oi00M4i.vj O lOOJ 0\v0 10 Oi vj 00 hi 4.. 0\*^ vO O tO U) Cn 0\ CO 
10 W OOU) 4^4i>.tOtA OiUi O^O^^3•v^00 OOU) vOU)vO-f»-"0-f»-V04>. 

M 4». On 00 



(ji4>--*^44.4>..*>.U)U)U)U)W to to (0 to w m i- m h m 
to >0 ^ l/l 4^ -1 vO On4». to m C» OnU) O vO OOUl U) hi Q "O v) On^^ U) tO 

4»' 00 H OOOi vOU)On 



00^ -^ "vl On On OnUt UiLn4'4».-t^U)UJU) 10 to tS m m m ih i- 

M-sjU) MvOUi MVlU) MvOCn O ONtO O 004^ O 00 On4». tO O 00 On-^ tO 

6o-vJ OnCa Ot4^WtOtOHH O OOvj vj OnOi 



Ii> ^! 2.,° 2 ^ °° °°^ ^ ^ "^^ Cn4^4>^4i.U) M to tO 
00 to OnU) -f" 00 to OnU) 04>.vOU)»^4^ muivO OnUJ 


^ 4^ 


H. oocn to 


H to to UJ ^ Oi ON'va 00 OOvO h to tOU)U)<-ncjicyi 


0\>J VI 


00 OOVO 
004^ VO Ul 


ON's-a 4>-4>.U)tO tO^P^OOO 00 00^ On OnUi 4i>- 4!>. U) U) 
t04». On000i0 4^ OnOOO t04>. OnOOO OstO 


to to to 
004«- 


On to 004>. 



UJU)U)U)U>U)U) to to to to to to 



ONUi 4>- U> to to O 



U)U)tOtOMtOtOtOtOtOH(-«lOH,HirH 

to M O OOVJ On*, to HI O O 00 On-^ OJ tJ hi O OC^ ON<-n 4». -^ U) 10 m 
vj *^ ^ ON00t0Oi'OU)4>- On04>.vj h 104^ OOtoWoiviO H t04>. OsOO 



4>.^4».4>.4^OJU)U)U)(Ot0t0t0l0Hi«MHiMM 

>«J4k. to H Ovjc^oj OVO OOCn U) M oovj 0\^ M O NO OOV] Ui 4». U> tO M 
HI *>1 U) H O OOU) O Ui 4«- U) OOUi to vO vj U\ M 00 Cn4>- tO O VO "^ Ln U) H 



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vO 4>- OOUi U) vj to On H 00U1 4». O 4>. h OOU) vj ^ tO O OnU) hi OOUt to 



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Ul HI OnO to 004>. vOUl OOH OstO OOMvavOUl hi4».vj OU)Ul 00H4^>^ 



OJU)UJtOtOtOlOtOtOtOIOHHiH,HHiHiHi 

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OU)U\ to OOOU; On 004^ OtOUlOOOONtOOl 004^ O OvtOOOl hivjOj 



Cn4>.4^4^4^4>-U)U)U»UJU> tO tO tO tO *' H 1-1 m hi hi 

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VI >J On On On OnC/i Ui4>.4>.4^4>-U)U)U)K)tOIOHHi„MM 
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O t»J O^vl NO tOUl 000 t04»v| OW ONV) vO (0 tn ON OOVO hi to 4>' U\ vJ 00 



> 



> 
w 



CHAPTER XII. 
COMPRESSED-AIR TRANSMISSION. 

The accompanying table, VII, gives the actual volume of 
air passing through a pipe of given diameter when the linear 
velocity of flow is known. This is merely a convertible 
table of pipe capacity, and will be useful as such in deter- 
mining the size of pipe best adapted for a given service, 
and it has nothing to do with the conditions which may 
determine the rate of transmission. 

While in considering the operation of air-compression we 
base our computations upon the volume of free air com- 
pressed, it is better in questions relating to the transmission 
of the air to consider the actual volume of the air during 
the transmission, or usually either at the beginning or at 
the completion of the transmission. As air in transmission 
soon attains the temperature of the pipe and its surround- 
ings, its temperature need not generally be taken into ac- 
count as affecting the volume. The volume of free air 
transmitted may be assumed to be directly as the absolute 
pressure or the number of atmospheres to which the air is 
compressed. Thus if the air transmitted be at 75 lbs., or 
6 atmospheres, the actual volume of free air transmitted 
will be six times the volume given in the body of the table. 
For comparing cases of transmission the linear velocity of 
flow is generally adopted, and is the more convenient form 
of statement. It is generally considered that for econom- 

IIO 



COMPRESSED-AIR TRANSMISSION, III 

ical transmission the actual velocity in main pipes should 
not exceed 20 feet per second. It would be well if more 
attention were given to the capacities of the distributing- 
pipes employed. In practice it often occurs that while the 
main pipe is large enough for the transmission, the smaller 
pipes, or hose, through which the air is finally transmitted 
to the individual machines are too small, and velocities as 
high as 50 feet per second are not infrequently met with. 

Compressed air has usually to be transmitted a greater or 
less distance from the compressor to the place where it is 
used, and in computing the cost or the economy of opera- 
tions in which compressed air is employed it becomes 
necessary to consider the friction of the air in the pipes, 
and the power lost in overcoming it. Upon this point 
some very extravagant and widely erroneous ideas have 
been quite generally disseminated. The popular impres- 
sion is that great power losses are inseparable from the 
transmission of air through pipes. The facts are quite dif- 
ferent from this. It seems to be certain that power maybe 
transmitted by compressed air for considerable distances 
with less loss than by any other known means. We can- 
not do anything for nothing, and of course there is some 
loss of power in the transmission of compressed air, but in 
general practice thus far, unless the piping system has been 
outrageously bad and inadequate, the losses by transmission 
have not been worth considering. The distances traversed 
have usually not been great enough to make the loss a 
serious one, or at all to be compared with the losses of 
power through the heating of the air in compression, or 
through its loss of volume by the cooling of re-expansion 
when the air was finally employed. But as compressed- 
air practice develops we want longer lines, and then the 



112 COMPRESSED AIR, 

question of transmission rises to greater importance. We 
want to convey air long distances for the purpose of em- 
ploying unused and now worthless water-powers. We 
want long lines of piping for supplying street-cars with the 
means of propulsion and for the general distribution of 
power and the general application of compressed air to its 
multifarious uses in large cities. We want to convey nat- 
ural gas from the wells where it flows to the towns and 
cities, where it may be used to the best advantage. 

When we come to look up the formulas to depend upon 
for our computations, we do not find any that are satis- 
factory and reliable. The best that may be said of the best 
of them, and that with caution, is that they are approxi- 
mately correct, but we know that they must be in error in 
some particulars. The available data in this line are 
meagre and chaotic, inconsistent and self-evidently unre- 
liable. It is perhaps too much to expect that close accu- 
rac ^can ever be attained in these computations. There are 
too many factors in the case, and a little variation in any 
one may make a great difference in the result. A state- 
ment of the friction in the case of compressed air flowing 
through a pipe involves at least all of the following factors : 
Unit of time, volume of air, pressure of air, diameter of 
pipe, length of pipe, and the difference of pressure at the 
ends of the pipe, or the head required to maintain the flow. 
Neither of these factors can be allowed its independent and 
absolute value, but is subject to modifications in deference 
to its associates. The flow of air being assumed to be 
uniform at the entrance to the pipe, the rate of flow is not 
constant for the whole length of the pipe, nor indeed for 
any point but the beginning of it. As the air may be said 
to carry in itself, in its elasticity, its own means of propul- 



COMPRESSED-AIR TRANSMISSION, II3 

sion, some of which it is using as it goes along, it is con- 
stantly losing some of its pressure, and its volume is there- 
fore constantly increasing. If the quantity of air entering 
the pipe is to continue to flow through it, the linear velocity 
of flow must be constantly accelerated on account of this 
increase of volume. This also modifies the use of the 
length of the pipe as a constant factor. It would be very 
natural to assume, as in the formulas in general use it is 
assumed, that if a certain head were sufficient to main- 
tain a certain flow for a given length of pipe, double the 
head would be sufficient for double the length. But that 
could not be so ; for in the second length all the head that 
propelled the air through the first length has disappeared, 
and the volume is now greater through the loss of that 
pressure, and the velocity is now greater, and it must require 
more additional head for the second length than was re- 
quired for the first. So of the other important factor, the 
diameter of the pipe. The actual area of section, ^^ the 
apparent capacity of the pipes, is, of course, directly as the 
square of the diameter, but the volume of air transmitted 
for given length and head will not be in any such ratio. 
The surface resistance of the interior is proportionately 
much greater in the smaller pipe. While the area is as the 
square of the diameter, the periphery is directly as the 
diameter. The greatest distance of any portion of the air 
from the periphery being less in the smaller pipe, the viscos- 
ity of the air counts for more. For volumes in proportion 
to the areas of the pipes a i-inch pipe will require for a 
given length more than three times as much head as a 2- 
inch pipe. 

Then besides the fluctuating values of these fickle fac- 
tors there is that other factor, unrecognized in our compu- 



114 COMPRESSED AIR. 

tations, but arrogantly assertive in practice — the condition 
of the pipe itself. The actual diameter of wrought-iron 
pipe, especially in the smaller sizes, is different from the 
nominal diameter. Some pipe is smooth, and some has 
seams and blisters. The pipe may be straight, or it may be 
crooked and have numerous elbows. Everybody knows 
that elbows are unpleasant things to encounter. Tables 
have been published of the effect of elbows in retarding 
the flow of compressed air. One of these tables, copy- 
righted, is before me, and from it I gather that eight or ten 
i-inch elbows have a retarding effect equal to one length 
of pipe, so that if the table is to be believed, elbows are not 
as obstructive as they are commonly supposed to be. If 
we say, without copyright, that a single elbow is equal to a 
length of pipe we will be nearer right than the table. 

No table or formula can make allowances for foreign 
substances or obstructions in the pipe, and it seems unnec- 
essary to call attention to the necessity of thoroughly blow- 
ing out the pipe before it is put to use. Long lines of 
pipe are sometimes laid through a variety of rough coun- 
try, and before the pipe is coupled up many things get 
into it that have no right to be there. In pipe-lines for 
transmitting natural gas it has been the practice before the 
pipes were put to service to turn on the full pressure of the 
gas and blow out the pipe. The pressure in such cases is 
often as high as four or five hundred pounds to the inch, 
and under such a force the pipe is usually quite effectually 
cleared, stones, sticks, leaves, squirrels, rats, and snakes 
having sometimes been ejected. 

And here it might be proper to say a word about the 
importance of the unimportant. It is the general practice 
of pipers when running a line of pipe for air or water or 



COMPRESSED-AIR TRANSMISSION, US 

Steam to put the white lead, or whatever may be used as a 
cement for the joint, into the coupling or elbow or other 
" female '' fitting, wiping it around and filling the threads 
with it, and then when the end of the pipe or the " male ** 
thread is screwed into it, none of the cement is left upon 
the outside, and a neat and clean-looking job is the result. 
The trouble in the case is that the job would not be so 
neat and cleanly looking if it could be seen from the in- 
side. As the pipe end is screwed into the fitting the ce- 
ment that does not remain in the threads — and not much 
of it does remain in the threads — is carried forward before 
the end of the pipe, and, when the pipe is screwed home, 
remains there and hardens, often rising above the inner sur- 
face of the pipe enough to cause a considerable stricture or 
reduction of pipe area. Now, if instead of this the cement- 
ing material is put upon the male thread when the pipe is 
screwed in, all that is not taken up by the threads remains 
on the outside of the pipe, instead of inside ; and although 
it does not look as neat as by the other practice, and en- 
tails the labor of wiping off the joint, we know that the in- 
side of the pipe is clear. 

FORMULA FOR THE FRICTION OF AIR IN PIPES. 

D = Diameter of pipe in inches ; 
Z = Length of pipe in feet ; 

V^= Volume of air delivered in cubic feet per minute ; 
If= Head or difference of pressure required to overcome 
friction and maintain the flow. 



ff= ^^^ 



lopooD^O' ' 



ii6 



COMPRESSED AIR, 



a/ \o,oQoD^aH 



Z>'^ = 



VL 



io,oooZr' 



z = 



lo^oooD^ all 



VALUES OF ^ FOR DIFFERENT NOMINAL DIAMETERS OF 
WROUGHT-IRON PIPE. 



1 " 35 

li" 5 

li" 662 

2 " 565 

2r 65 



3" 73 

si" 787 

4" 84 

5" 934 

6 " I 



8 
10 
12 
16 

20 

24 



I 

I 

• • k • o • I 

I 

. . r . . . I 
I 



2 
26 

34 

4 

45 



It will be noticed that the values of a for the i\" and 
the li" pipes are not consistent with the values given for 
the other sizes of pipe. This is in recognition of the actual 
diameters of those two nominal sizes of wrought-iron pipe, 
which are 1.38" and 1.6 1" respectively. 



// 



FIFTH POWERS OF D. 



n 



I ".. 


I 


3 ••• 


• 243 


li".. 


• • 3-05 


z¥'... 


• 525 


l\".. 


•• 7-59 


4"... 


. 1,024 


2 ".. 


.. 32 


5"--- 


• 3,125 


2r... 


• 97-65 


6 "... 


• 7,776 



>n 



10 



12 



20 



24 



n 



32,768 

100,000 

248,832 

3,200,000 

7,962,624 



COMPRESSED-AIR TRANSMISSION. II7 

Two or three examples are offered showing the applica- 
tion of the above formulas, although their use should be 
sufficiently evident to anyone capable of making the com- 
putations. The volume, F, is of course the actual volume of 
the air as it flows through the pipe under pressure, and not 
the volume of free air. 

Say that we wish to transmit 1200 cu. ft. of free air per 
minute at 75 lbs. gauge pressure, or 6 atmospheres, through 
a 4" pipe for 1000 ft., what additional pressure or head will 
be required to overcame the friction and maintain the flow 
of air ? 1200 cu. ft. of free air -^ 6 = 200 cu. ft. at 75 lbs. 
gauge. Then 



__ 200 X 1000 r lu 1. J -J 

H -= : — -r- = 4.65 lbs. head required. 

10,000 X 1024 X .84 ^ ^ 



Having a 4" pipe 1000 ft. long and a head of 5 lbs., what 
volume of air will be transmitted per minute ? 



rr i/iO)OOo X 1024 X .84 X "5 

V^yf — ^ = 207.38 cu. ft. 



1000 



The volume of free air in this case w411 be dependent 
upon the pressure during the transmission. If this 207.38 
cu. ft. were under a pressure of 60 lbs. gauge, or 5 atmos- 
pheres, the volume of free air would be 207.38 X 5 = 1036.9 
cu. ft. If the pressure were 90 lbs. gauge, or 7 atmospheres, 
the volume of free air would be 207.38 X 7 = 1451.66 cu. ft. 

Having 2000 cu. ft. of free air per minute compressed to 
100 lbs. gauge, through what length of 6" pipe may it be 
transmitted, losing 10 lbs. pressure in the transmission ? 
Here the terminal pressure would be 90 lbs. gauge, or 7 at- 



n8 



COMPRESSED AIR. 



mospheres, and the volume would consequently be 2000 -^ 7 
= 285.7 cu. ft. Then 

_ 10,000 X 7776 X I X 10 __ 

L — 2 = 9520 It. 

285.7' ^^ 

Having 1500 cu. ft. of free air per minute to transmit a 
distance of 2000 ft., the air being at 80 lbs. gauge, and wish- 
ing to deliver it at 75 lbs., what should be the diameter of 
the pipe? Here we have a head of 5 lbs., and the air is 
delivered at a pressure of 6 atmospheres, so that the deliv- 
ery-volume will be 1500 cu. ft. -f- 6 = 250 cu. ft. Then we 
have 

250' X 2000 



D'a 



10,000 X 5 



= 2500 in. 



This is the only case where the fifth power can possibly 
make any trouble for us, and by referring to the following 
table of values of D^a for the regular sizes of pipe the 
necessity of struggling with the fifth root is avoided. 



VALUES OF D^a. 



ff 



35 

1-525 

5-03 

2 " 18.08 

2*" 63.47 






99 



1774 

413-2 

4 " 860.2 



3i" 



5 
6 

8 

10 

12 

16 

20 

24 



2,918.75 

7,776 

36,864 

120,000 

313,528 

1,405,091 

4,480,000 

11,545,805 



• Our answer above being 2,500, we note that it is less than 
:^, 918.75, the value of D^a for 5" pipe, so that a 5" pipe 
will be a little larger than is required by the conditions, and 



COMPRESSED-AIR TRANSMISSION, 



119 



is the size of pipe that should be used. We may verify this 
by assuming a 5" pipe and computing what head would be 
required, the other conditions remaining unchanged. 



zr = 



250 X 2000 



10,000 X 3125 X .934 



= 4.28. 



As this head is somewhat smaller than 5, the given head, 
this also shows that a 5" pipe would be a trifle larger than 
would be required by the conditions, while a 4" pipe would 
be much too small. 

The pressures to which air is compressed do not in 
practice always, or generally, occur in even atmospheres. 
The following table, VIII, will be found convenient in as- 
certaining the actual volume of compressed air at any given 
pressure if the volume of free air is given, or vice versa. 

Table VIII. 

TABLE OF THE RELATIVE VOLUMES OF COMPRESSED AIR AT VARIOUS 

PRESSURES. 





Volume of 


Volume at 




Volume of 


Volume at 


Gauge 
Pressure. 


Free Air 

for I Cu. Ft. 

at given 


given Pressure 

for I Cu. Ft. 

of 


Gauge 
Pressure. 


Free Air 

for I Cu. Ft. 

at given 


given Pressure 

for I Cu. Ft. 

of 




Pressure. 


Free Air. 




Pressure. 


Free Air. 





I 


I 


45 


4.061 


. 2462 


I 


1.068 


.9356 


50 


4.401 


.2272 


2 


1. 136 


.8802 


55 


4-74 


.2109 


3 


1.204 


.8305 


60 


5.08 


.1967 


4 


1.273 


.7861 


65 


5.421 


.1844 


5 


1.34 


.7462 


70 


5.762 


.1735 


10 


1.68 


.5951 


75 ■ 


6.102 


.1638 


15 


2.02 


.4949 


80 


6.442 


.1552 


20 


2.36 


.4236 


85 


6.782 


.1474 


25 


2.7 


.3703 


90 


7,122 


.1404 


30 


3.041 


.3288 


95 


7.462 


.1340 


35 


3.381 


.2957 


100 


7.802 


.1281 


40 


3.72 


.2687 









120 COMPRESSED AIR, 

The second column in the above table gives the volume 
of free air for i cu. ft. of compressed air at a given pressure, 
and this value may be used as a multiplier for any number 
of cubic feet at given pressure to ascertain the equivalent 
volume of free air. 

Having 550 cu. ft. of air at 80 lbs. pressure, what will be 
the volume of free air ? 

550 X 6.442 — 3548.1 cu. ft. 

The third column in the table gives the volume of air at 
any given pressure for i cu. ft. of free air, and this value 
also may be used as a multiplier for any number of feet of 
free air to ascertain its volume after compression to a given 
pressure. 

If we have 1750 cu. ft. of free air, what will be its volume 
when compressed to 65 lbs. ? 

1750 X .1844 = 322.7 cu. ft. 

The following table, IX, of the head or additional press- 
ure required to overcome friction in the flow of air in pipes 
has been computed by the preceding formulae. It is be- 
lieved to be correct and reliable as far as it goes, and should 
be a convenience in many cases of compressed-air trans- 
mission for rock drills and similar uses. A table covering 
all the various pressures and conditions in general practice 
would be too voluminous to offer here. 

As we have before remarked, so many conditions may 
combine to modify the specific case of transmission that 
both the formulas and the table here given can have only a 
rough and general application and a provisional usefulness 
until something better appears. 



COMPRESSED-AIR TRANSMISSION: 



121 



Table IX. 

TABLE OF HEAD OR ADDITIONAL PRESSURE REQUIRED TO DELIVER 
AIR AT 75 LBS. GAUGE PRESSURE THROUGH PIPES OF VARIOUS 
SIZES AND LENGTHS. 

i-inch Pipe. 



Linear 

Velocity in 

Feet per 

Sec. 



12.72 

25.44 
38.16 

50.88 



Volume 

of 
Free Air 
per Min. 



25 
50 

75 

TOO 



Length of Pipe in Feet. 



50 



.245 
.981 
2.23 
3.925 



100 



.4944 
1.962 

4.45 
7.85 



150 



.735 
2.943 
6.68 
11.77 



200 



.98 

3.924 
8.9 
15.7 



300 



1.47 
5.886 

13.35 



500 



2.45 
9.81 



1000 



4.9 

19.62 



ij-inch Pipe. 



Velocity 

in 

Feet per 

Sec. 



6.7 
13. 4 
26.8 
40.2 



Volume 

of 
Free Air 
per Min. 



25 
50 

TOO 
150 



50 



.0567 

.2268 

.9072 

2.0412 



Length of Pipe in Feet. 



100 



.1134 

.4536 

I. 8144 

4.0824 



150 



.1701 

.6804 

2.7216 

6.1236 



200 



.2268 

.9072 

3.6288 

8.1648 



300 



.3402 

1.3608 

5.4432 
12.2472 



500 



.567 

2.268 

9.072 

20.412 



1000 



1 .134 

4.536 

18.144 



i^-inch Pipe. 









Length of Pipe in Feet. 


Velocity 


Volume 






in 


of 
Free Air 






Feet per 
















Sec. 


per Min. 


50 


100 


150 


200 


300 


500 


rooo 


4.9 


25 


.0172 


.0344 


.0516 


.0688 


.1032 


.172 


.344 


9.8 


50 


.068 8 


.1376 


.2064 


.2752 


.4128 


.688 


1.376 


19.6 


100 


.2752 


.5504 


.8256 


I . 1008 


I. 6512 


2.752 


5.504 


29.4 


150 


.6192 


1.2384 


1.8576 


2.4768 


3.7152 


6.192 


12.384 


39.2 


200 


I . 1008 


2.2016 


3.3024 


4.4032 


6.6048 


11.008 


22.016 



122 



COMPRESSED AIR. 



Table IX. — (Continued,) 
2-inch Pipe. 



Velocity 
in 


Volume 
of 






Length of Pipe 


n Feet. 






Feet per 


Free Air 
















Sec. 


per Min. 


50 


100 


150 


200 


300 


500 


1000 


6.369 


50 


.0192 


.0384 


.0576 


.0768 


.1152 


. 192 


.384 


12.738 


100 


.0768 


.1536 


.2304 


.3072 


.4608 


.768 


1.536 


19.107 


150 


.1728 


.3456 


.5184 


.6912 


1.0368 


1.728 


3.456 


25.476 


200 


.3072 


.6144 


.9216 


1.2288 


1.8432 


3.072 


6.144 


31.845 


250 


.48 


.96 


1.44 


1.92 


2.88 


4.8 


9.6 


38.214 


300 


.6912 


1.3824 


2.0736 


2.7648 


41.472 


6.912 


13.824 



2i-inch Pipe. 



Velocity 


Volume 


Length of Pipe in Feet. 


in 


of 
Free Air 




Feet per 
















Sec. 


per Min, 


100 


200 


300 


400 


500 


1000 


2000 


8.163 


100 


.0428 


.0856 


.1284 


.1712 


.214 


.428 


.856 


16.326 


200 


.1712 


.3424 


.5136 


.6848 


.856 


1. 712 


3.424 


24.489 


300 


.3859 


.7718 


I. 1577 


1.5436 


1.9295 


3.859 


7.718 


32.65 


400 


.6848 1.3696 


2.0544 


2.7392 


3.424 


6.848 


13.696 


40.81 


500 


1.072. 


2.144 


3.216 


4.288 


5.36 


10.72 


21.44 



3-inch Pipe, 



Velocity 


Volume 

of 
Free Air 


Length of Pipe in Feet. 


Feet per 


















Sec. 


per Min. 




















100 


200 


300 

.04941 


400 


500 


1000 


2000 


5.659 


100 


.01647 


.03294 


.06588 


.08235 


' .1647 


.3294 


II. 318 


200 


.06588 


.13176 


.19764 


.26352 


.3294 


.6588 


1. 3176 


16.977 


300 


.14823 


.29646 .44519 


.59292 


.74115 


1.4823 


2.9646 


22.636 


400 


.26352 


.52704 .79056 


T.054 


1-3176 


2.6352 


5.2704 


28.295 


500 


.41175 


.8235 1.233 


1.647 


2.058 


4.II75 


8.235 


56.59 


1000 


1.647 


3.294 4.941 


6.588 


8.235 


16.47 





COMPRESSED-AIR TRANSMISSION, 

Table IX. — {Continued^ 

3|-inch Pipe. 



123 



Velocity 
in 


Volume 

of 
Free Air 
per Min. 


Length of Pipe in Feet. 


Feet per 
Sec. 


100 


200 


300 


400 


500 


1000 


2000 


10.661 
21 .32 
31.98 
42.64 

53.3 


250 

500 

750 

1000 

1250 


.04202 
.16808 

.37817 
.67232 
1.0505 


. 08404 

.33616 

-75634 

1-344 

2.IOI 


.12606 
. 50424 
I. 1345 
2.0169 

3.1515 


.16808 
.67232 
I. 5127 
2.6893 
4.202 


.2101 
.8404 

1.89 

3-36 

5.25 


.4202 
1.68 
3.78 
6.72 
10.505 


.8404 
3-36 
7-56 
13.446 
21.01 



4-inch Pipe. 



Velocity 


Volume 


Length of Pipe in Feet. 


in 


of 
Free Air 




Feet per 
















Sec. 


per Min. 


100 


200 


300 


400 


500 


1000 


2000 


15.91 


500 


.08074 


.16148 


.2422 


.3229 


.4037 


.8074 


1.615 


23.86 


750 


.18166 


.3633 


.545 


.7266 


.908 


1. 816 


3.633 


31.82 


1000 


.32296 


•6459 


-969 


1.29 


1. 615 


3.229 


6.459 


39.77 


1250 


.5046 


1.009 


I-514 


2.018 


2.523 


5.046 


10.092 


47.73 


1500 


.7267 


1.4534 


2.18 


2.907 


3-633 


7.267 


14.534 



5-inch Pipe. 



Velocity 
in 


Volume 

of 
Free Air 
per Min. 


Length of Pipe in Feet. 


Feet per 
Sec. 


500 


1000 


2000 


3000 


4000 


5000 


10000 


10.18 
20.36 

30.54 
40.72 
50.90 


500 
1000 
1500 
2000 
2500 


.11896 

.4758 

1.0706 

1.9033 
2.974 


.2379 

-9517 

2.I4I3 

3.8067 

5.948 


.4758 
1.9033 
4.2826 

7-613 
11.896 


• 7137 
2.855 
6.424 
11.42 
17.844 


•9517 
3.8067 

8.565 
15.227 

23.79 


1. 189 

4.758 

10.706 

19.033 

29-74 


2.379 
9.516 
21.41 



124 



COMPRESSED AIR, 

Table IX. — {Continued.) 
6-inch Pipe. 



Velocity 

in 

Feet per 

Sec. 



14.18 
21.27 
28.36 
35.45 
42.54 



Volume 

of 
Free Air 
per Min. 



1000 
1500 
2000 
2500 
3000 



Length of Pipe in Feet. 



500 



.1786 
.4018 
.7144 
1. 116 
1.607 



1000 2000 



.3572 
.8037 
1.4288 
2.232 
3.215 



•7^44 
1.6074 

2.857 
4.465 
6.43 



3000 



I. 0716 
2. 411 
4.2S6 
6.697 
9.645 



4000 .5000 



1 .428 
3.215 
5.715 
8.93 
12.86 



1.786 

4.018 

7.144 

II. 162 

16.075 



10000 



3-572 

8.037 

14.288 

22.325 

32-15 



8-inch Pipe. 



Velocity 


Volume 






Length of Pipe 


in Feet. 






in 


of 
Free Air 






Feet per 
















Sec. 


per Min. 


1000 


2000 


4000 


6000 


8000 


10000 


15000 


15.91 


2000 


.296 


.592 


1. 184 


1.776 


2.368 


2.96 


4.44 


19.88 


2500 


.4626 


.925 


1.85 


2.775 


3.7 


4.62 


6.939 


23.86 


3000 


.6661 


1.332 


2.664 


3.996 


5.329 


6.66 


9.99 


31.82 


4000 


I.T84 


2.368 


4.737 


7.105 


9.474 


11.842 


17.76 


39.775 


5000 


1.85 


3 -701 


7.402 


II. 103 


14.8 


18.505 


27.757 



lO-inch Pipe. 



Velocity 


Volume 


Length of Pipe in Feet. 


in 


of 
Free Air 




Feet per 
















Sec. 


per Min. 


2000 


4000 


6000 


8000 


10000 


J5000 


20000 


10.18 


2000 


.1844 


.3688 


•5532 


.7376 


.922 


1.383 


1.8,44 


12.73 


2500 


.288 


.5763 


.8644 


I. 1526 


1.44 


2. 161 


2.88 


25.46 


5000 


1. 15 


2.305 


3.45S 


4.61 


5.763 


8.644 


II. 5 


38.19 


7500 


2.59 


5.186 


7.78 


10.37 


12.967 


19.45 




50 92 


1 0000 


4.61 


9.22 


13.83 


18.44 


23.05 







COMPRESSED-AIR TRANSMISSION. 



125 



Table I X . — ( Continued. ) 
i2-inch Pipe. 



Velocity 
in 


Volume 

of 
Free Air 
per Min. 


Length of Pipe in Feet. 


Feet per 
Sec. 


2000 


4000 


6000 


8000 


10000 


15000 


20000 


8.84 

1 17.68 

26.52 

I 35.36 
1 44.2 


2500 

5030 

7500 

1 0000 

12500 


.11075 

.443 

.9967 

1.772 
2.769 


.2215 

.886 
1.993 
3-544 
5.538 


•332 
1.329 

2-99 

5.316 

8.3 


.443 
1.772 

3.987 
7.088 
11.07 


-5537 
2.215 

4-98 
8.86 
13.84 


.83 I. 1075 
3-3224-43 
7-47 9-96 

13.29 17.72 

20.74 



CHAPTER XIII. 
THE UP-TO-DATE AIR-COMPRESSOR. 

The principal thing to be said of the up-to-date air-com- 
pressor is that it is not up to date. It would be difficult 
even now, and notwithstanding the improvements which we 
are told have been made in air-compressors in the last few 
years, to find the one that embodies the best knowledge of 
the time, or that in actual performance accomplishes what 
should be expected of it with our present knowledge of the 
practical conditions of economical compression. The 
standard of performance for a single-stage air compressor 
may be taken to be : a cylinderful of free air at normal 
temperature compressed isothermally, and ail delivered to 
the receiver, by an apparatus involving no losses through 
friction, and we should expect to realize a nearer approach to 
that standard than we do. We should in the first place be 
able to ascertain what is actually done in economical air- 
compression to-day, and if any one undertakes that he will 
find that it is no simple task. The catalogues of air com- 
pressor manufacturers are interesting in this connection, 
and the alleged indicator-diagrams contained in them are 
worthy of study. I have learned from them, if nothing 
else, to respect the wisdom of the builder who does not 
allow the diagrams from his steam- and air-compressing 
cylinders to be seen. 

While, as we know, air-compressors are built and running 
with the air-compressing cylinders placed tandem to the 

136 



THE UP-TO-DATE AIR-COMPRESSOR, 12/ 

Steam-cylinders, the piston rod of the steam-cylinder being 
continued into the air-cylinder and transmitting all the 
power required for compression directly to the compressing 
piston, and with a friction loss of only 5 per cent between 
the steam-cylinder and the air-cylinder, there are indicator- 
diagrams published in builders' catalogues that show very 
different results. In one set of diagrams, bearing every 
evidence of genuineness, but published without data, the 
ratio of the air-cylinder M.E.P. to that of the steam- 
cylinder is ^(iZTi') ^ loss of over 36 per cent in power alone, 
saying nothing of the other inevitable losses. In another 
catalogue a set of alleged indicator-diagrams is given with 
some accompanying data, and with a ratio of air-card to 
steam-card of .818, a loss of 18 per cent. A diagram from 
an air-compressing cylinder, published by another manu- 
facturer, shows the air-admission line above the atmosphere- 
line for almost the entire length of it, as though the air 
would rush into the air-cylinder with alacrity when the 
pressure was higher within the cylinder than outside of it! 
Still another builder, commenting in his catalogue upon this 
phenomenon, says that the fact that the air-admission line 
is above the atmosphere-line proves that his rival's piston 
leaks. I have in my possession still another indicator- 
diagram from a compressing cylinder with newly patented 
valves, and in which the air pressure in the cylinder at the 
beginning of the compression-stroke is ten pounds above 
the atmosphere, although the cylinder is filled with free air 
at each stroke and the entire compression is done in that 
one cylinder. And so it goes. We may say that the air- 
compressor builders are living upon the ignorance of their 
customers, or we may say that the blind are leading the 
blind, as may seem most correct fpr the individual case, 



128 COMPRESSED AIR. 

Of all the steam-actuated air-compressors in existence 
the one showing the very worst results, as far as economy 
of steam for the service performed is eoncerned, is the air- 
compressor used upon locomotives for operating the air- 
brakes. To compress a given volume of free air to a cer- 
tain pressure the " air-brake pump " uses nearly ten times 
as much steam as would be required in the best air-com- 
pressors of the day for the same service. The air-brake 
pump, however, is the one compressor whose extravagant 
waste of steam is condoned by the circumstances surround- 
ing its employment. There are more than 30,000 of these 
pumps in use, a number greater, perhaps, than that of all 
other air compressors combined, not counting those that 
are used for beer. While the wastefulness of this pump is 
fully conceded, its persistent use for air-brake service is 
completely vindicated. The pump is very simple and 
always ready, which is an important point, and the steam 
used to operate it upon the locomotive is mostly steam that 
otherwise would be blown off by the pop safety-valve. 
The pump is usually worked when stops are made at 
stations or when running down grade, and if the pump 
used much less steam it would generally mean not that so 
much steam was saved, but that the safety-valve would 
have so much more to do. Various styles of air-brake 
pumps have been devised showing a much better economy, 
but they have been successively abandoned for the estab- 
lished pump. It is only when the air-brake pump is used 
for the purpose of a general compressed-air supply, as it 
quite frequently is in railroad shops, that its extravagance 
is to be condemned. In such cases no language can be too 
severe to characterize the folly of it. That the air-brake 
pump can be used with profit and satisfaction to supply 



THE UP-TO-DATE AIR-COMPRESSOR. 



129 



compressed air for general use speaks highly of the value of 
the air. 

As a mechanical curiosity, and as exhibiting a great 
achievement of ingenuity, a set of indicator-diagrams from 
an air-brake pump are here reproduced, Fig. 20 being from 




Fig, 20. 

the steam-cylinder and Fig. 21 from the air-cylinder. The 
steam-cylinder diagrams are so different from the familiar 
cards of the ordinary steam-engine cylinder that it has been 
thought best to place the arrows upon them to indicate the 
direction of motion. They would look more natural to the 




Fig. 21, 

general steam engineer if he could be allowed to read them 
in the reverse way. It will be noticed that the steam 
pressure in the cylinder is low at the beginning of the 
stroke, corresponding with the low resistance in the air- 
cylinder, and that the steam pressure rises with the progress 
of the stroke, and at the end of it the cylinder is full of 



130 COMPRESSED AIR, 

high-pressure steam, while that steam has done much less 
work than would be due to the dead pressure of that volume 
of steam, saying nothing of the additional power that could 
have been developed by using expansively. This is evi- 
dently a more wasteful application of steam even than in the 
direct-acting pump for water. This distribution of steam, 
however, accomplishes the designed purpose of approxi- 
mately equalizing the steam pressure to the resistance, and 
the air-brake pump is thus enabled to dispense with the 
crank-shaft and all which it implies. 

Under any arrangement that has been invented for using 
steam economically the pressure in the steam-cylinder 
during the earlier part of the stroke is at its highest, and 
decreases generally to nothing, or nearly nothing, at the end 
of the stroke. In opposition to this the resistance in the 
air-cylinder at the beginning of the compression-stroke is 
very low and increases as the piston advances, and at the 
latter part of the compression-stroke this resistance is con- 
siderably higher than the force of the steam that is driving 
the piston. To keep the compressor in motion it is not 
enough that the mean effective pressure upon the steam- 
piston for the whole stroke shall exceed the mean effective 
resistance against the air-piston plus the friction of the 
entire apparatus. The force and resistance must be equal- 
ized in some way to keep up the movement, and various 
devices have been employed for this purpose. The usual 
reliance at the present time is upon the weight of the 
reciprocating parts and heavy fly-wheels, and it is doubtful 
still if there is anything that is in all respects to be preferred. 
A novel and ingenious arrangement for accomplishing this 
desired object has lately been brought out by one of the 
air-brake companies, not so much, it is understood, for air- 



THE UP-TO-DATE AIR-COMPRESSOR, 13 1 

brake service, as for general use in air-compression. The 
two air-cylinders of this compressor are horizontal and 
single-acting, and they together form the foundation for 
the entire compressor. While they are together equal in 
free air capacity to a double-acting cylinder of the sam@ 
diameter and stroke, they are in other respects quite differ- 
ent, as the pistons have movements independent of and 
always different in speed from each other, except momen- 
tarily at a point near the middle of each stroke. Above the 
air cylinders is placed the steam-engine, which forms a part 
of and which actuates the air-compressor. The engine 
comprises the usual elements of the horizontal steam- 
engine — the steam-cylinder and its piston, the cross-head, 
connecting-rod, crank-shafts, fly-wheels, and the mechanism 
of the valve motion. Short connecting-rods attached to 
the cross-head give motion to two compensating levers with 
changing fulcrums, and through these levers power is trans- 
mitted to the air-compressing pistons; and with a uniform 
movement assumed for the cross-head a continually decreas- 
ing movement is given to each air-piston for its compres- 
sion-stroke. At the beginning of either stroke of the steam- 
piston the fulcrum of the equalizing lever is above the 
middle of it, and the air-piston moves faster than the steam- 
piston. At the latter part of the stroke of the steam-piston 
the fulcrum of the lever is nearer its lower end, and the air- 
piston then moves much slower than the steam-piston. 
The indicator-diagrams Fig. 22 show the practical opera- 
tion of this compressor. The upper diagram, from the 
steam-cylinder, shows the steam at 100 pounds cut-off at 
four tenths of the stroke. The dotted line of the diagram 
shows the effect of the steam pressure for the stroke as 
modified by the weight and inertia of the reciprocating 



132 COMPRESSED AIR. 

parts. The lower diagram, from the c..r-cylinder, exhibits 




Am^an Machinist g^^^^ Cylinder 9 i» 10^ 

Air Cylinders 10^2 ^ ^* 
Fig. 22. 



the operation of compressing free air up to and delivering 



THE UP-TO-DATE AtR-COMPRESSOR, 133 

it at loo pounds pressure. The dotted lines in this diagram 
show the resultant force from the steam-piston as trans- 
mitted by the action of the compensating lever to the air- 
piston. It is evident that the work required of the fly-wheel 
in this case is less than in the ordinary steam-engine, while 
in the common air-compressor it is much greater. These 
cards show the friction of the compressor to be high, the 
ratio of the air to the steam-cylinder diagram being .75, a 
loss of 25 per cent from this source alone. 

The full sponsorial and patronymic appellation of the 
most pretentious member of the air-compressor family 
to-day is the Corliss Cross-Compound Condensing Com- 
pressor. It may be called the Five C's. The " cross '* is 
not practically as good as the tandem, but commercially 
the alliterative effect is valuable. The Corliss feature is 
one of the most valuable adjuncts for selling the com- 
pressor, but has nothing to do with operating it. The 
Corliss engine, as everybody knows, is designed to main- 
tain a uniform speed under a varying load. The cut-off 
controlled by the governor, is changed as the load changes 
and because the load changes. The air-compressor is 
required to maintain a constant air pressure when there is 
a varying demand for the air, and this varying demand 
means of course, and can only mean, a varying speed of 
operation, so that to take a fully equipped Corliss stationary 
engine and to attach an air-cylinder tandem to the steam- 
cylinder, or, if a double or compound engine, to attach an 
air-cylinder tandem to each steam-cylinder, the propriety of 
the arrangement must be very evident to those who can see 
it. All computations upon the efficiencies of air-com- 
pressors have been based upon the assumption of constant 
work under the best conditions. When it is recognized 



134 COMPRESSED AIR. 

that no compressed-air service is uniform in its demands, 
then the sacrifice of ideal conditions that the varying 
demand entails becomes quite an important factor in 
determining the ultimate economy of the system. How a 
compressor is governed is a very pertinent question for the 
economist. I cannot afford to go into it here, but I may 
say that nine tenths of all the air-compressors in use, not 
including the air-brake pumps, have no governors, and the 
governing devices employed upon most of the others are 
crude, unsatisfactory, and generally disgraceful. 

Where large air-compressing plants are to be established 
for continuous service, a much higher ultimate economy can 
be attained than where the plant required is not so extensive. 
It is best to use a number of units for the work of compres- 
sion instead of one or two large compressors. Air-com- 
pression offers little or no opportunity for the storage of 
power or for doing any work in advance, as may be done by 
a water-pump and reservoir. The receivers used in con- 
nection with air-compressors will not usually hold more 
than the compressor can deliver in one minute, so that if 
the demand for the air fluctuates it must be met by the 
speed of delivery at the compressors, and not by a change 
of reservoir supply. Air-compressors, like simple steam- 
engines, have their conditions of speed, pressure, etc., that 
secure the best economy; and where a plant consists of a 
number of units, all in operation, it will usually be more 
economical to let most of them, or as many as possible, run 
steadily at their best, and to do the governing or equalizing 
of the work by one or two of the compressors rather than by 
all of them. The more extensive the air-compressing plant 
may be or the more extensive the use of the air compressed 
the more uniform the demand may be expected to be. 



CHAPTER XIV. 
COMPRESSED AIR VERSUS ELECTRICITY. 

The title of this chapter is adopted in deference to the 
prevalent idea of the relations of these two power-trans- 
mitters. To my thinking the versus should be read as 
lightly as it is possible to read it, for there is in fact but 
little antagonism or competition between compressed air 
and electricity, and there is little likelihood that in practice 
there ever will be. Neither of them is a power-transmitter 
pure and simple, as a wire rope may be said to be, but each 
is capable of performing other functions, and the power- 
transmitting capabilities of each, in combination with their 
other individually peculiar lines of usefulness, open for 
each a distinct and separate field, which neither can fill for 
the other. The same is true of some of the other power- 
transmitters. They each have their special fields of useful- 
ness and adaptability which neither of the others could fill 
as well, if at all. 

Of late the gas-engine has been coming rapidly to the 
front as a valuable agent in the development, transmission, 
and distribution of power, and it has its enthusiastic advo- 
cates who are ready to predict that before long it is to 
supersede every other motor over a field that is practically 
boundless. But upon looking over the field a little farther 
and listening to another group of enthusiasts it soon appears 
that not the gas-engine but the oil-engine is the coming 

135 



136 COMPRESSED AIR, 

motor, and not only is it the coming motor, but it has 
already come, and is driving out the electric and gas and 
other motors, and the steam-engine also, in England and 
Germany and elsewhere in Europe, and it must soon do so 
also with us in the United States. But as we look into the 
operating conditions under which these several agencies 
may find employment we soon learn that each of the several 
motors is most applicable under conditions of its own, and 
that neither can do all that either of the others can do. 
Gas and oil, of course, develop power as the steam-engine 
does, while compressed air and electricity can only trans- 
mit power that originates elsewhere. But with the devel- 
opment and transmission of power the usefulness of gas 
(" producer " gas) or of oil ends, while with air and elec- 
tricity power-transmission is not their only function. 
Electricity has the vast field of illumination, in which it 
reigns supreme ; compressed air has no one application to 
compare in magnitude and importance with that of electric 
lighting, but it has a vast number of duties which are all its 
own, and which electricity cannot touch. The use of com- 
pressed air has been slow of development, and is still back- 
ward, but at this writing I am able to enumerate two 
hundred distinct and established uses of compressed air, 
and in more than 90 per cent of those uses electricity is 
absolutely inapplicable, and in the remainder, which form 
a field more or less open to other agencies besides either air 
or electricity, the air generally has the advantage. Turn to 
the last chapter of this little book, wherein some of the 
uses of compressed air are enumerated, and see all those 
that come under the first letter of the alphabet and judge 
where the competition with electricity comes in. In our 
list of the applications of compressed air some of the other 



COMPRESSED AIR VERSUS ELECTRICITY. I37 

letters of the alphabet develop a larger enumeration than 
the first letter, and the use of air for operating motors, or 
for producing rotary motion in general, or for performing 
any of the functions of the steam-engine, is not included. 
Referring to the portion of the list under the letter A^ it will 
be noticed that the only applications of air that compete 
with electricity are the air-brake and the air-hoist or the air- 
jack. The electric brake in competition with the air-brake 
is anything but a success, and it is not worth further men- 
tion. Even upon electric cars the air-brake is an absolute 
necessity for safety, and hundreds of lives have been sacri- 
ficed in our city streets because it has not been used. The 
air-jack also has the field to itself, and electricity is ^'not 
in it.'* In the field of general hoisting air and electricity 
divide the work, and the line of service done by each is gen- 
erally distinct from that performed by the other. There 
are establishments where they are thoroughly familiar with 
the uses and capabilities of electricity, operating, for in- 
stance, electric travelling cranes, and yet which use com- 
pressed air in numerous places throughout their works for 
hoisting, and where for the special services required electric- 
ity would have no chance at all. Where the direct-acting 
vertical hoist can be used, or the air-cylinder, either verti- 
cal or horizontal, with multiplying sheaves and a wire rope, 
it is, of course, preferable to electricity with its spinning 
motor-shaft, its drums and gearing. In the general work of 
hoisting as carried on at the Armour Packing Company's 
vast establishment electricity could not possibly do the work 
that the air does. The wonderful capability of standing 
ready for instant use at full power and without cost for 
maintenance for long periods of time seems to be possessed 
by compressed air alone. It is pre-eminently adapted to 



13^ COMPRESSED A/J^. 

uses that call for constant alertness, as in the switch and 
signal service and in the air-brake, and in the air-hoist it 
stands at its post day and night ready to give a lift the 
instant it is called upon. 

In the lines of service to which electricity and com- 
pressed air seem to be, perhaps, equally applicable, and 
where they could compete with no apparent disadvantage 
to either, it is to be regretted that circumstances seem 
invariably to defeat a fair comparison. In driving pumps 
a very fair test could be instituted of the relative merits of 
each, and of the losses in the use of each, as power-trans- 
mitters, and it happens that in this very work of pumping 
we find some striking illustrations of what might be termed 
the constant bad luck accompanying the air, or the malig- 
nant opposition of circumstances to any fair exhibition of 
its powers. Compressed air, by its very accommodating 
attitude, by its very applicability to widely varying con- 
ditions, is constantly placing itself in a false position before 
the community and showing itself at a disadvantage. It is 
able to accept conditions that enable it to make an un- 
seemly and unjust exhibition of its powers, yet which 
entirely exclude electricity from any such depreciatory 
performance. The pump that can be operated by electric- 
ity can be operated equally well by compressed air, the air- 
motor taking the place of the electric motor, either of them 
producing rotary motion ; and with suitable connecting 
gearing and with the pump mechanism unchanged one would 
have as good a chance as the other, and under those con- 
ditions the air could be made to do better than the elec- 
tricity. 

Electricity seems to be making advances more rapid than 
ever before in its employment for railway traction. It 



COMPRESSED AIR VERSUS ELECTRICITY. 13^ 

drives the horses from the surface roads, and is now be- 
ginning to supersede the steam locomotive. Perhaps all do 
not realize that this is the triumph after all of the steam 
engineer more than of the electrical engineer. Electricity is 
demonstrating not so much its superiority as a power-trans- 
mitter, but is simply showing the ultimate economy of gener- 
ating power in large central plants, even if the means of dis- 
tribution is a wasteful one, and accompanied by features that 
are insurmountably objectionable. The extending use of 
electricity as a railway motor is an argument also for com- 
pressed air, for it is able to take full advantage of the 
economy in centralized power development, and we are in 
the way to see very soon some practical demonstration of 
its abilities in this held. In railroad service, as in every- 
thing else, compressed air has been heretofore unfortunate, 
and its advocates and would-be promoters have wasted 
time and opportunity in developing minute economies in 
the air-motor which were not needed to enable it to com- 
pete with the best in the field. It may be regarded as cer- 
tain that whatever gain may be shown in the employment 
of electricity for traction its establishment is by no means 
a final solution of any question except of the economical 
generation of power, and that electricity has nothing to do 
with. 

Has any one called attention to the fact that one of our 
most prominent and perplexing political questions is entirely 
and indisputably the product of compressed air? Can 
electricity claim to have contributed any prominent factor 
in determining the course of parties or in shaping the 
destinies of the nation ? What if compressed air should be 
found to hold the balance of power and the deciding voice 
in the selection of a future President of the United States ? 



140 COMPRESSED AIR, 

This is the actual situation, and not an absurd or exag- 
gerated statement of it. The only political function 
attained by electricity is that of public executioner. Elec- 
tricity and compressed air stand to each other as the 
masked and nameless headsman upon the one side and 
Warwick the King-maker upon the other. The silver ques- 
tion of the day, whichever side of it w^e may find ourselves 
on, is entirely the outgrowth of the increased output of 
silver, and that, no one can deny, is what the air-driven 
rock drill has accomplished. The precipitation upon us of 
this perplexing question may have been a work of question- 
able beneficence, but the power that could achieve it is not 
to be treated lightly. 



CHAPTER XV. 
THE THERMAL RELATIONS OF AIR AND OF WATER. 

In all of our operations with compressed air, either in 
its compression, its storage and transmission, or in its final 
application to whatever purpose, the temperature of the air 
at any time, and the effect of raising or lowering its temper- 
ature by whatever means, are always important facts to be 
considered, and it will be well for us as early as possible to 
fix in our minds some general ideas upon the subject. The 
thermal relations of water are so different from those of 
air that by contrast a knowledge of the one may be made 
to enforce our knowledge of the other. The fact also that 
the effects of heat upon water are accepted as standards 
of heat measurements makes it necessary for us to know 
something about them. 

Say that we apply a given quantity or unit of heat to a 
pound of water, raising its temperature i degree ; hov/ 
much air would be equally heated, or have its temperature 
raised i degree, by the same unit of heat ? A cubic foot of 
air at atmospheric pressure — *' free air '* — and at 62 degrees 
weighs .076 pound, and a pound of air therefore in vol- 
ume equals i -^- .076 = 13.158 cubic feet. A pound of 
water is 27.7 cubic inches, and the ratio of volumes of 
water and of air of equal weight will be about i : 821. 
But, pound for pound, it takes less heat to raise the tem- 

141 



142 



COMPRESSED AIR. 



perature of air i degree, or any number of degrees, than is 
required to raise the temperature of water the same num- 
ber of degrees. The specific heat of water being i, that of 
air is only .2377, so that 13.158 cubic feet -^ .2377 = 55 
cubic feet, and this 55 cubic feet of free air is to be com- 
pared with I pound, or 27.7 cubic inches, of water. 

There is a means of fixing the thermal relations of air 
and water in the mind's eye so that they may not be easily 
forgotten. A common-sized glass tumbler, not quite full, 
holds a half-pound of water. A cubical box measuring 3 

feet each way, or say a large 
dry - goods box, holds, of 
course, a cubic yard, or 27 
cubic feet, which is, nearly 
enough, a half of our 55 cubic 
feet, so that the dry-goods box 
full of air and the tumbler 
full of water represent quite 
closely the volumes of air and 
of water that will be equally 
heated by equal units of heat. 
^*^- ^^' The approximate ratio of vol- 

umes will be I : 3431, and the ratio of the sides of two 
cubes representing the two volumes will be i : 15 +. The 
isometric projection of the two cubes here shown (Fig. 23) 
may convey and impress the relations better than the 
figures can do it. 

It is to be remembered that in the transmission of heat 
either to or from air or water — that is, whether heating or 
cooling them, or whether cooling or heating any bodies in 
thermal communication with them — the above ratios will 
prevail. Those whose attention is called for the first time 




THERMAL RELATIONS OF AIR AND WATER, 143 

to the phenomena accompanying air-compression or expan- 
sion cannot fail to be struck with the great changes of 
temperature that ensue coincidently with either operation, 
but the actual heat represented by these changes is usually 
overestimated, although circumstances that should check 
the exaggerated estimate are also at hand. If the body of 
the air-compressing cylinder and the cylinder-heads are 
properly water-jacketed, the temperature of the air deliv- 
ered is considerably lower than it would be if there were 
no water-jacketing, but, at the same time, the perceptible 
heating of the water in the jacket, by which the partial 
cooling of the air is effected, proceeds quite slowly, show- 
ing that the actual quantity of heat abstracted from the air 
by that means is not great. So also when the heated com- 
pressed air flows through pipes for some distance, the rapid- 
ity with which its temperature approaches that of its envi- 
ronment is another evidence of the small amount of heat 
actually carried by it. Still we hear constantly of the won- 
ders of heating or cooling that are to be done by compressed 
air— wonders that never fully materialize with a more ex- 
tended experience. In the Pohle ** air lift pump " ( which 
is not properly a pump, as it has absolutely no moving or 
working or wearing parts, but which is a very valuable ap- 
plication of compressed air direct for raising water from 
bored wells, and where the air is discharged upward into the 
submerged end of a vertical water-pipe, the air entering the 
pipe, distributing itself through the water, and rising with it, 
expanding as it rises) it is claimed that the expansion of 
the air while in contact with the water cools the water. 
We may say that the expanding air certainly does cool the 
water, and we may also say that it certainly does not cool 
the water more than a fraction of a single degree. 



144 COMPRESSED AIR, 

Where an establishment is equipped with a permanent 
compressed-air plant, for operating hoists, jacks, presses, 
and isolated machines of all kinds, it is a simple matter to 
rig up an arrangement for cooling the drinking-water for the 
employes. Take a \\" pipe loo feet long (50 feet might 
be long enough), place it horizontally, and connect one end 
of it to the compressed-air supply, with a suitable cock to 
control the escape of the air. Leave the other end of the 
pipe open and enclose the whole of the pipe, after passing 
the air-admission cock, in a thick non-conducting covering. 
If nothing better is at hand, take plenty of paper, and wind 
it on spirally layer after layer, covering the whole pipe. 
Then lead a %" water-pipe into the open end of this hori- 
zontal released air-pipe, and let it come out by a tee or 
otherwise at the other end of the air-pipe, and the whole 
apparatus is provided. The air in this case should be thor- 
oughly cooled and have all its suspended water discharged 
before its release in the cooling-pipe. 

A touching spectacle in all our large cities are those mel- 
ancholy monuments of a futile philanthropy, its public 
drinking-fountains. The motive that prompts their erec- 
tion is worthy of all respect. Much money has been ex- 
pended upon them with the best of intentions, but with the 
poorest of results. They can nowhere be said to be a suc- 
cess, for they do not accomplish what they propose to do. 
They offer the cup to the thirsty lip, but it is practically 
an empty cup, for it does not hold what we want. Who 
wants to drink warm water ? The most costly and artistic 
of fountains is nowhere, in the thought of the hot and 
thirsty crowd, in competition with a bucket of cold water 
and an old, rusty tin dipper. The instinctive call of hu- 
manity for cold water to drink is so absolutely universal 



THERMAL RELATIONS OE AIR AND WATER, I45 

that it must be correct, and should be more adequately 
provided for. 

Our cities are constantly doing more and more for the 
comfort and well-being of all the people. To all of us life 
is more worth the living by reason of our co-operation and 
our collective helpfulness. Amid all that is designed to 
make our cities more attractive and more desirable to live 
in, is there any possible material thing that can be sug- 
gested more to be desired, more proper to do, more promis- 
ing of universal good, than to make it possible for every 
man, woman, and child to have always at hand a drink of 
cold water ? Does not compressed air make it possible ? 
The establishment of a general compressed-air service in 
any city might be gloriously celebrated by the establish- 
ment of a cold-water fountain. 

If anyone does undertake to cool drinking-water by the 
use of compressed air, we may expect to hear that it takes 
a great quantity of air to cool a little water, which is just 
about what I have been writing above. The case, how- 
ever, in the matter of cooling by compressed air is not 
nearly as bad as I seem to make it appear. The actual 
heat represented by any change of temperature in air com- 
pressed to a pressure of several atmospheres is of course 
greater than for air compressed to only i atmosphere, or 
free air, as we call it, and directly in proportion to the re- 
spective absolute pressures. With air at 75 lbs. gauge, or 
6 atmospheres, the same change of temperature in either 
heating or cooling would indicate a transfer of six times the 
amount of heat that would be indicated by the same heat- 
ing or cooling of free air. The sudden release of air com- 
pressed to 6 atmospheres and at 62° before the release 
would cause a theoretical fall of temperature of over 200°, 



146 COMPRESSED AIR. 

and if this air were in communication with water that re- 
quired to be cooled but 20°, this would give the air a con- 
siderable advantage. In the case of the Pohle air lift 
pump, cited above, where the volume of water would prob- 
ably be as great as that of the compressed air in contact 
with it, the cooling effect of the expanding air could be but 
slight, as before stated. 



CHAPTER XVI. 
THE FREEZING UP OF COMPRESSED AIR. 

The most familiar and the most constantly reiterated 
objection to the use of compressed air is its well-known 
habit of " freezing up " under certain conditions, and too 
many who have not sufficiently investigated the subject 
have regarded this freezing-up of the air as an insurmount- 
able and fatal objection to its use for purposes for which it 
would seem to be otherwise eminently adapted. By " the 
freezing up of the air/' as the expression is commonly used, 
— although, of course, it is never the air that freezes, — we 
understand a deposition of moisture, more or less rapid, 
upon the sides of the pipes or passages that convey the air, 
and its accumulating and freezing there until the area of 
the channel is materially reduced, the proper flow of the 
air prevented, and the operation of the air-motor or other 
apparatus seriously impeded or stopped entirely. This 
phenomenon may easily occur and has frequently occurred 
in the use of compressed air. The earlier experimenters in 
this line all encountered it, and most of them on account 
of it at once dropped compressed air as a practicable power- 
transmitter, and the freezing up of compressed air has 
remained a formidable bugaboo among otherwise intelligent 
mechanics to this day. 

The best way to do in a case like this is first of all to 
have a good look at it all around in broad daylight. It 

147 



148 COMPRESSED AIR, 

would seem to be worth while to get together where we can 
see them the principal facts of the case, so that we may be 
able to understand the conditions under which the freezing 
up occurs, whether it -must always accompany the use of 
compressed air, and, if not, the combination of conditions 
under which all danger of freezing up may be successfully 
avoided. 

Intelligent mechanics to maintain their up-to-date in- 
telligence must be wide-awake and fully informed, and such 
should know that in these days compressed air is widely 
used not only without freezing up, but also without any 
reheating or other special device for preventing it. Com- 
pressed-air locomotives, probably hundreds of them, are 
constantly used, in mines and elsewhere, without any re- 
heating of the air and without freezing up, and the builders 
of those locomotives will absolutely guarantee them to do it 
every time. Rock drills by the thousand and pumps and 
hoisting-engines without number are run by compressed air 
without reheating it and without freezing up. 

It must be evident that for freezing up to occur two 
things are essential, and neither alone could have any effect 
toward producing such a result. The free moisture must 
be present and accumulating, and the temperature of the 
air where the freezing up is to occur must be below the 
freezing-point. The moisture alone can cause no trouble 
as long as the temperature continues high enough. It 
will simply be carried along with the air and will be dis- 
charged with it. So, too, a low temperature of the air in 
the passages at any time will not freeze up anything as 
long as there is no free moisture present to be frozen. 

We may say generally that air always contains moisture. 
Its capacity for moisture is determined by the combined 



THE FREEZING UP OF COMPRESSED AIR, 1 49 

conditions of pressure and temperature to which it is at the 
time subjected. Changes either of pressure or of tempera- 
ture immediately change the capacity of the air for water, 
and, supposing the air to be saturated with water, whenever, 
either through increase of pressure or through decrease of 
temperature, the capacity of the air for water is reduced, 
the excess of water is dropped. At constant temperature 
the capacity of air for water seems to be inversely as its 
absolute pressure. By another mode of stating this it may 
be said that the capacity of the air for water is independent 
of its pressure or density. It is so stated by parties who 
have an eminent right to speak upon the subject ; and the 
statement is correct if rightly understood, but it is apt to 
be misleading. At uniform temperature a given volume of 
air implies a capacity for a certain weight of water, whether 
the air be at a pressure of i atmosphere or of loo atmos- 
pheres; but if the air has been compressed from a press- 
ure of I atmosphere to a pressure of loo atmospheres, or 
if its volume has been reduced from, say, loo cubic feet 
to I cubic foot, or in that proportion, its capacity for 
water has really been reduced to one hundredth of its 
original capacity; and if the air before the compression 
was saturated with water, then after the compression, and 
after it has fallen to its original temperature, it must have 
dropped somewhere during the operation -^^ of the water 
that it originally carried. 

At whatever pressure the air may be changes of temper- 
ature immediately affect the capacity of the air for carrying 
water. The water-carrying capacity of the air seems to be 
as sensitive to temperature as to pressure. We know very 
distinctly the general fact that the hotter the air the greater 
its capacity for moisture ; but there seem to be little satis- 



I so COMPRESSED AIR. 

factory data as to the quantity of water that will be carried 
by compressed air under different conditions of tempera- 
ture. The absence of such data, however, need not 
seriously cripple us in our quest. 

In the operation of air-compression the heating of the 
air, and the increase of water capacity thereby, seems to 
keep pace with and to compensate for the reduction of 
water capacity consequent upon the reduction of volume, 
and we never hear of any trouble from liberated water in 
the compressing cylinder, but after the air leaves the com- 
pressor the water begins to make itself known, and all the 
world hears of it. As the air leaves the compressor it is 
usually quite hot, and even at the high temperature the air is 
usually saturated, or nearly saturated, with water. As the air 
cools the water begins at once to be released, and before it 
is thoroughly cooled considerable water is generally depos- 
ited. Changes in the meteorological conditions, or in the 
original humidity of the air as it enters the compressing 
cylinder, of course change the amount of water precipitated 
by the air after compression, and all who have experience 
with compressed air find that on this account the air carries 
and deposits more water at some times than at others. 

Many amateurs and experimenters have encountered 
trouble from the freezing up of the air on account of taking 
the air immediately from the compressor, before it has been 
completely cooled, or, if cooled, by neglecting to drain off all 
the liberated water from the pipes before using the air. The 
experience thus obtained embodied an important lesson if 
it could have been learned, but the lesson has been too 
often misread, and the interpretation of the freezing up 
phenomenon has been an incorrect one. When an air- 
motor or an engine driven by compressed air "freezes up/' 



THE FREEZING UP OF COMPRESSED AIR. 151 

usually by the choking of the exhaust passages, the general 
impression among mechanics is that the water is precipi- 
tated by the air at the moment when the freezing occurs ; 
but the fact usually is that the water is deposited in the 
pipes by the air before the motor or engine is reached, 
and the water is then carried along as entrained water by 
the friction of the air, and when the temperature of the air 
falls below the freezing-point, on account of its expansion in 
the cylinder or at the exhaust, the water, being present and 
in contact with the cold air, is of necessity frozen. 

The general practice of the day in the compression and 
transmission of air does not seem to make adequate provi- 
sion for disposing of the water deposited by the air while 
cooling. As the air leaves the compressor it is usually 
quite hot, and even at the high temperature it is saturated 
or nearly saturated with water. As the air cools it begins 
at once to lose its capacity for water, and some of the water 
is dropped and continues to be deposited as long as the air 
continues to cool. In connection with the compressor, and 
usually quite near it, a receiver or reservoir of considerable 
capacity is provided, the most important function of which 
is, or is assumed to be, that of collecting the water that may 
be precipitated by the compressed air. In too many cases 
this receiver fails of its mission, or only partially collects the 
water from the air, because, if the compressor is working 
constantly and rapidly, as it usually does, the air goes 
through the receiver and out of it and into the pipe-line 
before it has time to cool. The air after compression will 
not drop all of its water until it is thoroughly cooled, and 
the cooler it gets the greater will be the quantity of water 
liberated ; and when the air, still under full pressure, has 
reached the lowest pressure attainable, means should then 



152 COMPRESSED AIR, 

be provided for collecting the liberated water, or it must, of 
course, be carried along in the pipes to make trouble 
by freezing up where the air is used, and where the air 
expands and cools while doing its work or upon its release. 
With a receiver near the compressor, and with hot air passing 
through it, and a pipe-line long enough to completely cool 
the air before it is used in rock drill, air-motor, pump, or other 
constantly running machine, and with no provision for dis- 
posing of the water in the pipe, we should expect to hear of 
the machines freezing up. Cases are quite common where 
a second receiver placed at the farther end of a pipe-line has 
effectually cured the freezing up by removing the congeal- 
able liquid. 

A few years ago many air-compressors for driving rock 
drills were in use in the United States — a large number of 
them upon the Croton aqueduct — which cooled the air dur- 
ing compression by the injection of jets of water into the air 
in the compressing cylinder. Compressors of this style are 
not now built by any firm in the United States. The cylinders 
were found to wear out quite rapidly, the compressors could 
not be run as fast as the dry compressors, and for other 
similar reasons they did not pay. They did, however, deliver 
the compressed air decidedly cooler than the compressors 
now in use deliver it, and it is not surprising that it should 
be claimed by rock-drill runners, and probably correctly, that 
those old injection compressors, with the water intimately 
mingling with the air during the compression, still furnished 
drier air, and consequently air less liable to ** freeze up," 
than the more modern dry compressors furnish. 

In the process of wood-vulcanizing, for preserving wood 
by cooking the sap in the wood, the material to be treated 
is enclosed in tight cylinders and subjected to an air press- 



THE FREEZING UP OF COMPRESSED AIR, 1 53 

ure of 150 pounds The air is specially heated and made 
to circulate around and among the wood and absorb the 
moisture that may be liberated from it, so that it is essential 
to the process that the air should be as dry as possible, and 
the paradox occurs that to secure dry air the wet or injec- 
tion type of compressor is employed. 

I know a certain iron mine which has two air-compressors 
side by side, each connected to deliver the compressed air 
through the same receiver and the same pipe to the rock 
drills in the mine. One of the compressors delivers its air 
at a temperature considerably below that of the air from 
the other compressor, say from 50° to 100° lower, neither 
of the compressors being of the injection type, and it is con- 
stantly noted that the men in the mine operating the drills 
can immediately tell which compressor is running by the 
relative humidity of the air supplied. The compressor which 
delivers the coolest air of course delivers the driest air. 

When the air is completely saturated with water, contact 
with water will not make it any wetter. The water in the 
injection compressor did not wet the air, for it was as wet 
as it could be ; and as that water enabled the compressor to 
deliver the air at a lower temperature than the dry com- 
pressor would deliver it, the air, simply because it was 
cooler, actually emerged from the compressor bearing less 
moisture than the air emerging at the same pressure, but 
at a higher temperature, from the dry compressor. If no 
means were provided for draining the surplus water from 
the air, except, in either case, the receiver located near the 
compressor, the cooler air passing the receiver would carry 
the less amount of water into the pipe and through it ; but 
if, in each case, after the air had traversed the pipe a suffi- 
cient distance to have become thoroughly cooled another 
receiver or drainage chamber had been provided, there is 



154 COMPRESSED AIR. 

no reason why, after emerging from the chamber, the air in 
each case being at the same pressure and temperature, the 
one should carry any more water than the other. To get 
rid ot all trouble from water in the air, and the possible 
freezing of it, care should be taken that when the air passes 
a point where it is still at full pressure and has reached 
its lowest temperature, such means of drainage shall be 
provided that none of the liberated water shall be carried 
into and along the pipes beyond that point. 

The possible freezing up that we have been contemplat- 
ing thus far along in this chapter is where water is present 
by deposition from the compressed air, and where a low 
temperature is caused by the expansion of the air, and the 
freezing of the water ensues by contact. Another mode of 
freezing up is experienced where the freezing is accom- 
plished not by the air that has been compressed, but by the 
external atmosphere. In the winter if compressed air at 
low temperature, but still above the freezing-point, saturated 
with water, as it is pretty sure to be, and with the pipe 
thoroughly drained to a certain point, has then to pass for 
some distance through a pipe exposed to a freezing atmos- 
phere, it cannot fail to deposit some water, and the freez- 
ing of the water so deposited may soon choke the pipe. I 
have encountered cases of this kind more than once, notably 
in one of the largest chemical works of the country, where 
the pressure of the air is employed in lifting and transferring 
acids so that they may not be exposed to metallic contact. 
The air-pipe was carried through the extensive works and 
from building to building, and in some places between the 
buildings where exposed to the extreme cold of a sharp 
winter it was choked up by the accumulation and successive 
freezing. The only apparent lesson in this case is to protect 
the pipe from frost. A pipe conveying compressed air and 



THE FREEZING UP OF COMPRESSED AIR, 155 

exposed to a freezing atmosphere is quite sure to choke up. 
The deposition of the water may proceed slowly, but if the 
low external temperature continues, the accumulation will 
eventually reduce the air-channel, or even close it entirely. 
This result is, of course, chargeable to the weather, and not 
to the innate frigorific malignity of the compressed air. 

The pressure at which the compressed air is transmitted, 
and eventually used, has an important bearing upon the 
question of its freezing up in use. If the air is transmitted 
only short distances, and at comparatively low pressures 
the probabilities of freezing up are much greater than if 
high pressures are employed and if the distance of trans- 
mission is at least sufficient to allow a thorough cooling and 
drainage of the air while under the full pressure. In the 
use of low-pressure air for any service, of course a larger 
volume of free air is used to furnish any given power, and 
the larger volume of air implies the presence of a greater 
quantity of water in suspension, and the lower pressure em- 
ployed affords less opportunity, or no opportunity, for ex- 
tracting the water, and, as a fact of experience, most of the 
freezing-up trouble that is encountered is from air that is 
used at comparatively low pressure. 

There are many considerations, which I need not enu- 
merate here, to commend the use of air at high pressures, 
and not the least among those considerations is the practical 
immunity from freezing up that is thereby secured. This 
may be readily understood Say that air is compressed to 
looo pounds gauge, or, say, 70 atmospheres, either that 
smaller pipes may be used for long-distance transmission, 
or that smaller receivers may be used for the storage of air 
upon a street railway motor, and say that the air is admitted 
to the motor at 100 pounds pressure. If while the air is at 
1000 pounds it is thoroughly cooled and drained, it is evi= 



156 COMPRESSED AIR, 

dent that when that air is expanded to 100 pounds, and 
has been allowed to regain its normal temperature, if the 
air was just saturated with moisture when at 1000 pounds 
pressure and normal temperature, when it has expanded to 
more than eight times its former volume it can be only one 
eighth saturated, and no water can be deposited by it in 
expanding from 100 pounds downward, and however low 
the temperature may fall there can be no freezing up. 

A valuable use of compressed air is for the transmission 
of packages or mail matter through suitable tubes from one 
station to another. In this pneumatic transmission service 
some trouble has been experienced in the winter from the 
accumulation of ice in the pipes. As the pressure employed 
is low the freezing up might be prevented by compressing 
all the air to a pressure considerably higher than required 
and cooling and draining it while under that higher pres- 
sure. Then after passing a reducing valve to the low pres- 
sure for use the air would be dry and could not deposit 
moisture to be frozen. 

Compressed air is often used in caissions for bridge piers 
and kindred uses where the compressor is so near the cais- 
sion that the air in transmission does not become as cool as 
it should be, and the men find the warm atmosphere very 
oppressive and are unable to do as much work as should be 
expected. The service pipes are sometimes cooled by pass- 
ing them through water, but even then the air in the caissons 
is warmer than it should be for vigorous work. In this case 
also if the air were compressed to a pressure, say, 20 or 30 
pounds higher than required, then cooled as well as possible 
by passing the pipes through cold water, and after that ad- 
mitted to the caisson through a pressure -reducer adjusted 
to the desired pressure, it would then be cool enough, or it 
might even be made cooler than required. 



CHAPTER XVIL 
REHEATING COMPRESSED AIR. 

While air at low temperature has a comparatively small 
cooling effect upon water or upon whatever may be in con- 
tact with it, the fact inversely applied is of advantage in the 
use of compressed air for power-transmission. It requires 
comparatively little heat to raise the temperature of air 
rapidly. It is well known that after the transmission oi 
compressed air to the point where it is to be em.ployed a 
considerable saving in the cost of the available power is 
effected, theoretically at least, by reheating the air before it 
is used. While many have called attention to this matter 
in various ways, few have given us any definite and reliable 
data regarding it. Little is generally known as to the actual 
economy of such a practice, or of the conditions under 
which it is practicable 

It may easily be shown that where a certain volume of 
air has been compressed to a given pressure, and has by 
transmission or storage resumed approximately its normal 
temperature, if that air is then reheated and thereby ex- 
panded, the additional volume of compressed air resulting 
from the expansion is produced by an expenditure of heat 
much lower than the original volume of compressed air was 
produced for, and by a much lower expenditure of heat 
than is required to produce an equal volume of steam. The 
actual figures in the case, all theoretical, are as follows : 

157 



158 COMPRESSED AIR 

Weight of I cu. ft. of steam at 75 lbs gauge =. 2089 lb 

Total units of heat in i lb. of steam at 75 lbs from 
water at 60^ =1151. 

Total units of heat in i cu ft of steam at 75 lbs = 1151 
X. 2089 = 240.44. 

To produce by compression through a steam-actuated 
air-compressor i cu ft of compressed air at 75 lbs. and 60^ 
about 2 cu. ft of steam of the same pressure are required. 
or the heat-units employed in producmg i cu. ft of com- 
pressed air will be about 240.44 X 2 = 480 88 heat units as 
the thermal cost of i cu. ft. of compressed air at the above 
temperature and pressure The temperature and volume 
of the air as it leaves the compressor will be considerably 
higher than the figures here assumed^ but as the air is in- 
variably stored for a time, or is transmitted through pipes 
to a distance between its compression and its ultimate 
employment, it may be said to always return to its normal 
temperature before it is used, so that, whatever we may 
have at the compressor, the air as it is delivered to the 
motor, or whatever apparatus may be operated by it, will 
have cost, as above stated. 480.88 heat-units for i cu. ft at 
75 lbs The difference m the thermal cost of any volume 
of compressed air thus produced by mechaniral compres- 
sion and the cost of any additional volume of air that may 
result from the subsequent reheating of the air is very 
striking. 

The weight of i cu ft of free air at 60^ = 076 lb. 

Weight of X cu. ft- of compressed air at 75 lbs. and 
60°= 456 

Units of heat required to double the volume of i lb of 
air at 60° = 123 84. 

Units of heat required to double the volume of 1 cu ft 



REHEATING COMPRESSED AIR. 1 59 

of compressed air at 75 lbs. and 60° = 123,84 X .456 = 

56.47. 

Cost of I cu. ft. of superheated compressed air at 7 5 lbs 

compared with the cost of i cu. ft. of compressed air as 

produced by ordinary compression: 

480.88 : 56.47 ; : I : .1174. 

Here we see that the cost in heat-units of the volume of 
air produced by the reheating is less than one eighth of the 
cost of the same volume produced by compression. Upon 
this showing the matter is certainly worth looking into, 
because if there is such a possible opening for the econom- 
ical application of heat to the development of power, we 
ought to know it and avail ourselves of it 

The operation of reheating compressed air is correctly so 
termed. It is, m fact, a case of doing work over again, or 
of replacing in the air heat that has been lost by it in previ- 
ous operations. It must be confessed that the presumption 
is all against our finding much profit in this direction 
There are not many places m life where it pays to do our 
work a second time There is, as we have seen, practically 
no air-compression without heating the air by the operation, 
and there is no transmission of air after compression with- 
out its cooling to very near its original temperature If the 
air could go immediately from the compressing cylinder into 
the motor cylinder, where it does its work, without losing 
any of its heat, it would have the same effective power as 
it would have after long-distance transmission and cooling 
and reheating, and without the additional cost of that re- 
heating. While we are saying in all good faith that there 
is little loss of power in the transmission of compressed air 



l6o COMPRESSED AIR. 

to considerable distances, and that the difference in the 
pressure of the air at the two ends of a long pipe necessary 
to overcome the friction and maintain the flow is but smal), 
and that it is, to a great extent, compensated for by the 
increased volume at delivery, the fact still is that there is a 
great loss of power in the transmission of the air, if we 
reckon from the moment when compression ceases on ac- 
count of the inevitable cooling of the air. Still this loss is 
not properly chargeable to the transmission, for no matter 
how far the air may be transmitted the cooling is all accom- 
plished before the air has travelled very far if the pipes are 
of proper size. Supposing air to be transmitted ten miles, 
it must be conveyed with considerable rapidity if it does 
not get down to normal temperature before the end of the 
first quarter of a mile. 

As the volume of air under any constant pressure varies 
directly as the absolute temperature, it follows that to 
double the volume by heating the air its absolute tempera- 
ture must be doubled. The air being at 60°, its absolute 
temperature will be 60 + 461 = 521, and double this will 
be 521 X 2 = 1042, the absolute temperature required. 
This by the thermometer will be 1042 — 461 = 581°. As 
this is the temperature that is required for the air when 
delivered into the motor, and actually beginning its work, 
it will be necessary, on account of the ease and rapidity 
with which it cools, to heat the air considerably higher than 
this theoretical temperature. It is one thing, and an easy 
one, to heat the air, while it is a very different and a very 
difficult thing to keep it hot. To avoid all loss of heat it 
would be necessary, not only to keep the pipe which con- 
veyed the air constantly hot, but also the cylinder in which 



REHEATING COMPRESSED AIR. l6l 

it was used, or it would be cooled before it began to do its 
work. In one case within my experience, where com- 
pressed air was reheated, and its absolute temperature was 
increased at the heater 38 per cent, and where, of course, 
its theoretical increase of volume was the same, the actual 
increase of power realized was only 12 per cent. In this 
case the air was transmitted after the reheating about 20 
feet, the pipe was not covered, and no precautions were 
taken to prevent loss of heat by radiation. The volume 
of air transmitted was sufficient to develop between 20 
and 30 horse-power. The theoretical temperature re- 
quired to double the volume of compressed air at 60" being 
581°, the actual temperature required at the heater under 
the most favorable conditions in order to have a double 
volume of air available in the motor will not be less than 
800°, and this is a temperature that it is practically impos- 
sible to employ and maintain, and we may as well give up 
all thought of doubling the volume of cooipressed air by 
reheating it and of realizing the promised economy of the 
operation. 

If instead of doubling the volume we only attempt to 
increase it by one half, or 50 per cent, which it is practica- 
ble to do, the required theoretical temperature (absolute) 
will be 521 + 50 per cent = 782, and 782 — 461 = 321°, 
the sensible temperature required. Adding enough to this 
to allow for the intermediate cooling, the actual temperature ^ 
required should probably be not less than 450°. The tem- 
perature of the air before the reheating being assumed to be 
60°, the increase of temperature will be 450° — 60° = 390°- 
As we saw above that it required 56.47 heat-units to raise 
the tempeature of i cu. ft. of compressed air at 75 lbs. 
gauge pressure from 60° to 581°, the actual increase of tem- 



1 62 COMPRESSED AIR, 

perature being 581 — 60 = 521, it follows that to raise the 
temperature 390° will require : 

521 : 390 : : 56.47 : 42.27. 

Then if the first cubic foot of compressed air costs 480.88 
heat-units for its compression, and if the additional half of 
a cubic foot produced by reheating costs 42.27 heat-units, 
the total cost of i^ cu. ft. under the reheating system will 
be 480.88 + 42.27 = 523.15, and the cost per cubic foot at 
this rate will be 523.15 ~ i^ = 348.76 heat-units. The 
relative cost in heat-units of i cu. ft. of compressed air pro- 
duced by compression alone, and of a cubic foot resulting 
from compression and reheating, will be : 

480.88 : 348.76 : : I : .72. 

From this it appears that the gain by reheating com- 
pressed air sufficiently to increase its effective volume 50 
per cent will be 28 per cent. The more fair and correct 
way to state this, however, will be to reverse it : 

.72 : I : : I : 1.38. 

We may say, then, that the total fuel applied with the 
reheating system will yield 38 per cent higher results than 
are to be realized without the reheating. This seems to be 
very near the maximum that can be attained in the way of 
economy by reheating dry compressed air. 

It is not always, nor, indeed, often, that the reheating of 
compressed air is practicable or possible. In a valuable 
report upon compressed-air appliances by a committee of 
the Master Car-Builders' Association, 1894, they say: "It 
was reported by the manufacturers of air-appliances that 
superheated compressed air used in air-lifts, jacks, engines, 



REHEATING COMPRESSED AIR, 1 63 

etc., increases the efficiency fully 50 per cent, but your 
committee was unable to make tests or to procure reliable 
data, etc." The ''manufacturers of air-appliances,'* quoted 
by the committee, either were not responsible for their 
words or they did not know what they were talking about. 
Bearing in mind the facility and rapidity with which heated 
air in transmission loses its heat, it is idle to think of ever 
heating compressed air except for continuously running 
motors, and then by heaters very close to the motors. In 
Paris, where 25,000 horse-power is employed for general 
compressed-air service, the air in some instances is used to 
run engines that were formerly run by steam, the original 
boiler that supplied the steam for the engine being retained 
as a heater and reservoir for the air. That is all right, and 
wherever any motor or engine is to be run without inter- 
ruption a heater for the air should certainly be employed ; 
but at the end of this volume is a list of two hundred dif- 
ferent and distinct uses of compressed air in not one of 
which would it be practicable or anything but a losing op- 
eration to try to heat the air. In the United States at the 
present time there is probably not one case in a thousand 
where compressed air is employed and where one cent of 
profit could be realized from reheating the air. It is to be 
regretted that American compressed-air practice is not so 
far developed, or developed upon such lines, as to make the 
economy of reheating the air before its use more readily 
and generally available. When, by and by, compressed air 
comes to be used for what we may term legitimate power- 
transmission, and is employed to drive small motors and 
motors not so small with the established functions of the 
steam-engine, then the reheater will find its field of useful- 
ness. 



164 



COMPRESSED AIR, 



In connection with this topic it is hoped that the accom- 
panying diagrams, Figs. 24 and 25, will be of some interest 
and value. Fig. 24 shows the increase of volume accom- 



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constant during the operation represented. The relative 



REHEATING COMPRESSED AIR. 



165 



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horizontal line above that indicating, successively, an 
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1 66 COMPRESSED AIR. 

Fahrenheit, and the figures above the upper line indicate 
the corresponding absolute temperatures. 

Fig. 25 shows the increase of pressure only caused by 
the heating of compressed air, the volume being constant. 
The air is assumed to be heated, as in Fig. 24, from the 
several initial temperatures of 0°, 32°, 60°, and 100°, and 
also from a number of different initial pressures. The 
pressures are indicated by the several horizontal lines, the 
vertical distance between any two adjacent lines represent- 
ing approximately i atmosphere. The figures at the left of 
the diagram indicate the gauge pressures, and the figures at 
the right the absolute pressures. The temperatures are in- 
dicated as in the previous diagram. 



CHAPTER XVIII. 
COMPRESSED AIR FOR PUMPING. 

The air-compressor owes its modern development to the 
demands of the rock drill, in its various forms and appli- 
cations, more than to any other single cause. The larg- 
est builders of air-compressors for general use first engaged 
in their manufacture to supply the air to the rock drills 
that they were building. All of the rock drills in all of 
the mines, and in every tunnel that is being driven, and in 
every shaft that is being sunk, we may say, are and appar- 
ently must be driven by compressed air. Nobody seems to 
inquire, and nobody knows very clearly, whether or not the 
power employed in operating rock drills is applied economi- 
cally or not. The conditions under which the drills are 
employed and the nature of their work are all inimical to 
economy. It is impossible to measure the actual work 
done by a rock drill. It is enough that their work pays, 
and that there is little annoyance or anxiety involved in 
keeping them going all right. 

But, taking the mines as they run, the operating of the 
drills is only one of several uses of power in mining opera- 
tions, and not the largest of these, although to some it may 
seem to be the most important. Hoisting, including haul- 
age, and pumping, either of them, upon the average, re- 
quires more power than is required for the drilling. Good 
steam-engines at the surface may generally be employed 

167 



1 68 COMPRESSED AIR, 

for the hoisting, and they do so well that we need not here 
trouble ourselves much about them. With the use of 
power for mine-pumping it is all very different. The 
pumps must generally be located where the water is, and, 
if steam is used to drive them, far away from the boiler 
plant ; and it so happens that to-day probably the most 
wasteful use of steam to be found anywhere is to be found 
where mine pumps are operated. 

This certainly ought not to be so. The operation of 
pumping is one of the most favorable ever found for the 
economical application of power. The marine-engine and 
the water- works pumping-engine divide the honors as exam- 
ples of the highest economy in the wide range of modern 
engineering practice. The stationary engine, except under 
the most favorable conditions, does not equal their per- 
formance. The success of the marine-engine and of the 
pumping-engine is to be attributed to the one operating 
condition that they have in common, and that is not found 
elsewhere. They have constantly uniform work to do. 
The pumping of water from our mines should also be eco- 
nomically done, because in this service, almost as much as 
with the water-works pump, the height of the lift is practi- 
cally constant and unvarying. It is not necessary to say 
how different is the actual result in the case of the mine 
pump. Waste of power and expense for repairs, and for 
duplicate machinery, are the natural and inevitable accom- 
paniments of steam-pum.ping in deep or extensive mines. 
The trouble of course is principally in carrying the steam 
so far. 

Everybody should know that steam need not be employed 
under such conditions. Compressed air stands ready to do 
this work and to do it more cheaply and more satisfactorily 



COMPRESSED AIR FOR PUMPING. 1 69 

than it can be done by any other means. It is to be con- 
fessed that air, where it has been employed for mine-pump- 
ing, has not by any means made as favorable an exhibit as 
it should have made, and has not made the progress that it 
should toward universal adoption for this service. The 
why of it is easily found. 

Here is a practical example in the case of what is said to 
be the largest coal-producing mine in the United States. 
Of course they have a lot of pumping to do there, and of 
course they use for it the always handy steam-pump. In 
this mine the steam is conveyed to the pumps by one line 
about a mile, and by another line about a mile and a half. 
The steam condenses all along its journey, and with high 
pressure at the boilers there is low pressure, and often too 
low pressure, at the pumps. What starts from the boilers 
as steam is mostly water when it gets to the pumps, and 
they are operated by combined hydraulic and vaporous ac- 
tion, the simple steam-pump tlius becoming what might be 
termed a diabolically reversed compound. The great feat- 
ure of the American steam-pump is that it will actually go. 
That it will go in the mine makes it also go in the market. 
It is to be recorded to its credit (?) that it makes it possible 
to do what it should be impossible to do. And so we find 
in this mine eight of these pumps, in various grades of in- 
efficiency, and the steam is carried a mile or a mile and a 
half to make them go. The steam leaks everywhere, the 
roofs are rotting and tumbling in under the combined ac- 
tion of the heat and the moisture, sometimes joints blow 
out or pipes break, men are scalded, passages are blocked, 
ventilation is stopped, gangs of repairers are constantly em- 
ployed, but the troubles keep increasing. Something has 



I70 COMPRESSED AIR. 

to be done about it. It will not do to use steam any 
longer here. 

When it comes to this point it is very unfortunate that 
this is a coal mine. A little item in the cost of a plant to 
be installed will outweigh all considerations of fuel or of 
power economy. The decisions of coal-mine managers are 
therefore no suitable precedents for the miners who must 
feed their boilers with greenbacks. Electricity or com- 
pressed air, either of them, stands ready to take up this job 
of pumping, and get rid of this steam nuisance in the mine. 
But electricity has no chance here, because new pumps 
would have to be bought, and there would be also the wir- 
ing of the mine, while compressed air is so accommodating 
that it agrees to use the old pumps and the old pipes, and 
no one need doubt that the pumps will actually go. Com- 
pressed air is accordingly adopted, not upon its merits, but 
because it will not cost so much to put it in ; and electric- 
ity is not permitted to make an unseemly exhibition of 
itself. 

Few perhaps realize how peculiar, and, indeed, unique, 
have been the conditions under which electricity has been 
spread abroad. In every case where it has been employed 
in power-transmission its installation has involved an en- 
tirely. new plant throughout, each end of the plant has been 
adapted to the other, and every detail to the whole, so that 
it has never been placed in a false position or shown at a 
disadvantage, as compressed air is almost invariably served. 
If electricity had secured a chance at this mine, putting in 
the new electrically-driven pumps, as well as the generators, 
and with everything new, consistent, and complete, it would, 
no doubt, have put on airs over the results accomplished, 
as compared with what compressed air has done in some 



COMPRESSED AIR EOR PUMPING. I /I 

cases ; but there would have been, after all, really no basis 
for comparison. 

For supplying the compressed air instead of steam to this 
mine, an excellent compressor-plant is installed, a plant 
that any manufacturer might be proud of and might be 
pardoned for bragging about. The performance of these 
compressors is presumably as good as that of any compres- 
sors to be found to-day. But the air furnished by the com- 
pressors is to be used in those abominable steam-pumps. 
I have elsewhere expressed my disgust at, and protested 
against, the apparent indifference of the air-compressor 
builders as to the uses to which the air is put after it leaves 
the compressor, or as to whether the air is applied econom- 
ically oi not. Too often the explanation in the case is the 
same as in this case, completely exonerating the compres- 
sor builders. The conditions determining the adoption of 
compressed air for this mine are that the old pumps are to 
be used. Of course the compressor builders cannot afford 
to kill their business to save their ideals ; they get a good 
job, and the compressors are put in to drive those pumps. 

It is not known that any worse contrivance has yet been 
discovered, as far as power economy alone is concerned, 
than the common direct-acting steam-pump. There are 
enormous clearances to be filled at every stroke of the 
pump, without any compensation for the waste or any justi- 
fication of its existence, except the very insistent one that 
the clearance is one of the conditions necessary to make 
the pump go, or is necessary when tired steam is used. 
Not the slightest advantage can be taken of the possible 
expansion of the steam or air. Too often the reverse of 
expansion occurs, and at the termination of the stroke the 
cylinder is filled to a higher pressure than was required to 



172 COMPRESSED AIR, 

make the stroke, and all the cylinderful to be immediately 
exhausted. This may be worse with a duplex than with a 
single pump, as either piston may reach the end of its 
stroke and the cylinder may then be overfilled with steam or 
air while it is waiting to have its valve reversed by the move- 
ment of the other piston. But it always happens, in addi- 
tion to this, that the pumps are not proportioned to the 
work to be done, or that the several pumps are not all so 
proportioned that the same pressure will operate each of 
them. Where the pumps were driven by steam trans- 
mitted a long distance, it might have been well to calculate 
upon lower steam as the distance increased, and to plan the 
pumps accordingly, but there is no calculation of the kind 
undertaken. The pumps are simply bought ready-made, 
and they fit about as well as other ready-made goods. 
Take the published list of the pumps in the mine that we 
are talking about. The actual heads under which the sev- 
eral pumps are operated is given, and I have added lo per 
cent to the pressures due to those heads, and the operating 
pressures required in the steam-cylinders of the several 
pumps are then as follows ; 



No. I 


2 


3 


4 


5 


6 


7 


8 


Lbs. 32 


28 


10 


31 


39 


23 


37 


32 



No. 3 is a small pump and runs buc a short time each 
day, and is not of much account to us. Nos. 4, 5, and 6, 
however, are the largest pumps employed, located near 
each other, and delivering under the same head. If the 
above are not the several pressures required, they must be 
nearly in these ratios, and they seem to complacently ig- 
nore the fact that all the pumps should be operated by 
approximately the same pressure, especially if compressed 



COMPRESSED AIR EOR PUMPING, 1/3 

air is used to drive them. The pump for which the lowest 
pressure is sufficient, running under throttle, is quite likely 
to fill its cylinder with air at the highest pressure before the 
exhaust occurs. Unless the piping is outrageously inade- 
quate the air pressure will be practically the same through- 
out the mine. In this mine the pipe capacity is liberal, 
and it is safe to assume that the air pressure in the pipes 
supplying these eight pumps will not be found to vary more 
than I lb. or 2 lbs. at the most throughout the series. 

At this writing, the air-plant having been in operation 
considerably over a year, I have the written word of the 
superintendent that its efficiency has not been to this day 
definitely ascertained. All of the pumps have not been 
operated at once except in emergencies. The most defi- 
nite statement obtainable is that with the compressors at a 
certain speed six of the eight pumps are run at once, which 
tells us nothing, as we do not know^ the speed of the pumps. 
It would be a simple matter, when everything was going, 
and at any time agreed upon, to station a man at each 
pump and let him count the strokes, and this, in connec- 
tion with the revolutions of the compressors, would tell us 
much. From a knowledge of all the available data in this 
case, and from some knowledge of similar cases, I am will- 
ing to hazard the assertion that not more than 20 per cent 
of the I.H.-P. at the steam-cylinders of the compressors is 
to be found in the weight of water delivered by the pumps. 

Of course the arrangement as it stands is a great improve- 
ment over the use of direct steam at the pumps, and every- 
body is to be congratulated over the change. Not only 
is there an actual reduction in the total consumption of 
steam, where it is used in the cylinders of the air-compres- 
sors instead of in the cylinders of the pumps, but all of the 



174 COMPRESSED AIR. 

annoyance, delay, danger, and expense of the steam-distri- 
bution is avoided. That, with suitable pumps, a different 
air pressure in the pipes, and a consistent combination of 
machinery throughout, the same work could be done for 
one half of the fuel is not worth considering, for this is a 
coal mine, you know. But if the same work could have 
been done with one half of the boiler and compressor- 
plant, that would been worth considering even at a coal 
mine, and is deserving of much serious thought where the 
installation of additional plants is under consideration. 

Something may of course be said upon the other side of 
this case, and toward shifting the responsibility for it. 
Mine managers are not pump experts. The attitude of the 
pump builders is similar to that of the compressor builders. 
They simply sell the pumps and they know little about how 
people may employ them, as I have been told by agents and 
salesmen of pump establishments. No special pumps are 
built to be operated by compressed air. There should be 
such pumps in the market, and compressed air should not 
be used with any thought of economy in the common direct- 
acting steam-pump. The pump should be a geared pump, 
and the air-motor should be an engine with a cut-off 
adopted to the pressure of air employed. 

Right here seems to be offered a fine opportunity for 
comparison between electricty and compressed air for 
power-transmission. Pumping is a line of work that either 
may do, and with little apparent unfair advantage in the 
conditions. The same pumps that are being put in to be 
operated by electricity would be equally adapted to be opera- 
ted by compressed air, by the substitution of an air-engine 
for the electric motor and an adjustment of the gearing to 
correspond, and a fair comparison of the results might be 



COMPRESSED AIR FOR PUMPING, 1/5 

made. In a Western town a pumping-plant has recently 
been installed to be driven by electricity. The pumps are 
bought by the town, and the local electric-lighting company 
undertakes to maintain and operate them, transmitting the 
current 2000 ft., for 4 cents per 1000 gals, delivered against 
a pressure of 60 lbs., which is about twenty times the fuel 
cost for the same work in the best pumping-engines of the 
day. Electricity might sublet this contract to compressed 
air for one quarter of the figure, and the air would be 
greatly inflated over its good luck in getting the job. 

In the general work of pumping there is evidently a great 
field still to be occupied by compressed air. It is the 
3iatural power-transmitter, and incomparably the best, for 
mining operations ; and as the mine pumping requires 
more power than the rock drills, more compressed ail 
should be used in our mines for the pumping, while, as 
a matter of fact, probably not one quarter as much air is 
used, and steam or mechanical transmission of power is 
employed at great inconvenience and expense. A pressure 
of 6 atmospheres, which is very suitable for the rock drills, 
could also be used to good advantage for the pumps, and 
proper arrangements for cooling and draining the air would 
fully dispel all danger of freezing, which is the prevalent 
bugbear of compressed-air practice. 

Besides the pumping for mines there is the constantly 
recurring problem of power-transmission for the water* 
supply of towns and cities, and compressed air is well 
adapted for such service. With a general compressed-air 
service established in our large cities the air would be ready 
to operate the thousands of pumps, now driven mostly by 
isolated hot-air engines, which supply the tanks upon the 
roofs of high buildings, or of buildings on ground too high 
for the established water-supply to reach, ^ 



CHAPTER XIX. 

A LIST OF THE VARIOUS APPLICATIONS OF 
COMPRESSED AIR. 

This list is intended to include only the direct applica- 
tions of compressed air to specific uses, and not its employ- 
ment in an air-motor, or where it takes the place or does 
the work of a steam-engine or other power-developer. The 
list is of course incomplete, as such a list must necessarily 
be, for the applications of compressed air develop faster 
than they can become generally known and recorded. A 
slight explanation of the way in which the air is used is 
given in a number of cases. 

Acids^ Raising or Transferring, — Compressed air is 
largely used for this purpose in chemical works, or where 
acids are handled in bulk or in large quantities and where 
contact with the metals cannot be allowed. Vessels con- 
taining the acid are subjected to a pressure of air inside 
them and above the acid, and the pressure of the air causes 
the acid to flow wherever the pipe may lead it. 

Accumulator for Hydraulic Hoisting Service. — The com- 
pressed-air accumulator takes the place of the heavy weights 
long used for maintaining a uniform and constant pressure 
and regulating the supply of water necessary in operating 
hydraulic cranes, lifts, etc. The air, compressed to the 
pressure required to be maintained, is contained in an up- 
right cylindrical vessel of considerable capacity. The water 

176 



A LIST OF THE VARIOUS APPLICATIONS. 1 7/ 

rises and falls in the lower part of the vessel, and a consider- 
able fluctuation of level is possible without great change in 
the air pressure. A float upon the surface of the water 
controls the movement of a duplex pump to maintain the 
required water-supply. 

Aerated Bread. 

Aerated Fuel. — A jet of compressed air vaporizes or 
atomizes crude petroleum in furnaces which have a wide ap- 
plication in the arts wherever great heat with perfect control 
is required, as in glass factories, brick or lime kilns, forges 
and metal works, etc. The system is also used for gener- 
ating steam, but for that purpose is not generally cheaper 
than coal. Lamps using compressed air with oil in a similar 
way are much used for out-of-door work, also in rolling- 
mills, railroad yards, etc. 

Aerating Molten Metal in the Bessemer Process. — l^his 
was a revolutionary application of compressed air, and of 
untold importance in the manufacture and in the promotion 
of the use of steel. The air is forced up through a mass of 
melted cast iron, burning out the carbon and in a few 
minutes converting the entire mass into steel, thus produc- 
ing steel cheaper than iron. 

Aerating Water, — The aeration of water is usually carried 
on in connection with its filtration, and is equally necessary 
in many cases to render the water wholesome and potable. 
Extensive works for the purpose are provided in connection 
with the water-supply of many towns and cities. The air 
is made to traverse a series of water-tanks, passing succes- 
sively up through the contents of each, carrying off volatile 
and objectionable constituents and imparting the necessary 
oxygen. 

Agitating Syrups in Sugar-refineries. 



178 COMPRESSED AIR, 

Air-brake. — The air-brake, in use upon all passenger 
trains, and also largely used for freight, puts the control of 
the train entirely with the engineer. Before the use of 
compressed air for this purpose the brakes upon each car 
were applied and released separately by individual brake- 
men upon steam-whistle signals from the engineer. The 
brakeman is discharged and the whistle is seldom heard. 
The air is compressed by an air-brake pump upon the 
locomotive, and there are over 30,000 of these air-brake 
pumps in use, a number greater, perhaps, than that of all 
other air-compressors together. The air-brake pump has 
begotten a great number of new applications of compressed 
air, especially in connection with the different departments 
of railroad service. 

Air-brake upon Street Railways. — The value of the air- 
brake upon the steam-roads and the necessity for a quicker 
and more efficient brake for street-cars, now that the cable 
and the trolley have made them heavier and have increased 
their speed, are leading rapidly to the adoption of the air- 
brake for street-railway service. They air-compressing 
pump on the street-car is operated by a crank or eccentric 
upon one of the axles of the car. 

Air^ Dense, see Dense-air Refrigerating. 

Air-hoist. — This term is used in contradistinction to the 
pneumatic crane, the crane generally employing drums and 
gearing or other complicated mechanism, while the move- 
ment of the hoist is simple, direct, and of limited range. 

Air-jack. — Air-jacks are largely used in railroad shops 
and are a distinct outgrowth of the air-brake pump, the 
pump being always at hand or easily procurable with other 
railroad supplies, and may be readily piped up wherever it 
may be w^anted. The jacks are more properly air-lifts, usu- 



A LIS 7^ OF THE VARIOUS APPLICATIONS. 1 79 

ally operating from below the load to be lifted. Many- 
jacks are sunk in specially prepared pits under repair tracks 
for taking out wheels and axles. Portable jacks are in use 
which have wheels and handles like a barrel truck. Stand- 
ing the truck up sets the lifting cylinder upon its base, and 
the jack is at once ready for work. The air pressure is 
supplied by an air-hose connected with a convenient branch 
upon the air-supply pipe. " Pulling down *' jacks are made 
for pulling down defective sills upon freight cars. 

Air-lift Pump, — The Pohle air-lift pump is not prop- 
erly a pump, except that it is employed for raising water, 
and it has no working or moving parts of any kind. A 
vertical water-pipe, usually in a bored or artesian well, ex- 
tends down some distance below the level of the water to 
be lifted, and at the lower end of it, which is open, a com- 
pressed-air pipe discharges the air upward into the column 
of water, and the mingled air and water rise and flow from 
the upper end of the water-pipe. The flow of water con- 
tinues as long as the air is supplied. The pump gives ex- 
cellent economical results and is highly commended. 

Air-lifts^ see Elevators. 

Air-lock Doors. — The air-lock is used for the ingress and 
egrees of workmen and material to and from caissons, the 
excavating chambers of soft-ground tunnels, or wherever 
operations are carried on under air pressure. The lock is a 
chamber of sufficient size to receive two or more men at a 
time or a bucket of material. It is provided with two sets 
of doors and valves to admit or discharge the air. To 
enter the working chamber the outer door of the lock 
is opened, then the men enter the lock and this door 
is closed. A valve is opened admitting air from the 
working chamber until an equal pressure is attained in 



l8o COMPRESSED AIR. 

the lock, when the inner door may be opened and the men 
admitted. The same operation occurs in the admission 
of material, and the process is reversed for egress. By a 
late improvement the outer doors of the air-lock are opened 
and closed by the pressure of the air acting upon pistons 
connected with the doors^ and the locks are operated more 
rapidly than formerly, especially for the hoisting or lowering 
of material. 

Applying Hose-couplings. 

Armor^ Divings see Diving-armor. 

Asphalt-refining, see Refining Asphalt. 

Automatic Pump, see Ejector. 

Automatic Fire-extinguisher, 

Balloon, Water ^ see Raising Ships. 

Beatifig Eggs. 

Beer-pump. — The use of compressed air for forcing beer 
from barrels, for the retail trade in saloons and hotels, must 
be more extensive than its use for the air-brake. The air 
pressure for this service is either provided by hand power 
or automatically by hydrant pressure. 

Bell-ringing, see Ringing Bells. 

Bellows, Organ^ see Organ Bellows. 

Blacksmith' s Fires. — Where a compressed-air supply is 
maintained for operating rock drills or general machinery, 
and at a pressure of 6 or 7 atmospheres, the air for blow- 
ing the blacksmith's fire is sometimes taken from the com- 
pressed-air pipes. This is a costly way of supplying air 
at such a light pressure. The power required for com- 
pressing each cubic foot of free air for the rock drill is 
probably ten times as great as would be required for the 
blowing pressure. 

Blast, Sand, see Sand-blast. 



A LIS 7^ OF THE VARIOUS APPLICATIONS, l8l 

Block Signal^ see Switch and Signal Service. 

Bessemer Process^ see Aerating Molten Metal. 

Boiler-shop, — Compressed air is now capable of supply- 
ing all the power required for operating boiler-shops, ma- 
chines or apparatus, mostly portable, being provided for all 
of the operations involved. Hoisting, punching, shearing, 
drilling, tapping, reaming, riveting, chipping, caulking, and 
screwing in and cutting off stay-bolts are all quickly, effi- 
ciently, and economically done by compressed air. 

Brake^ Air, see Air-brake. 

Bridge -building, — Compressed air is a valuable assistant 
in bridge-building, both in the preparation of the material 
in the shop and in the erection of the structure. In the 
shop the air is used for hoisting and in portable tools for 
drilling, reaming, riveting, chipping, etc., as in the boiler- 
shop. The erection of the bridge would be in many cases 
impossible without the compressed air in the caissons for 
the piers, while in the work of erection the portable tools 
are called in again. 

Caisson, — The caisson is used principally for excavations 
under water, and subsequently for building, in place of the 
material removed, bridge piers, or solid masonry for any 
purpose. The caisson is essentially an open box, of a shape 
corresponding to the purpose desired, closed at the top and 
loaded above until it sinks where the work is to be done. 
The caisson being open at the bottom, the water is excluded 
by the maintenance of an air pressure within, the pressure 
increasing with the submergence until at a depth of 80 or 
100 feet the limit of human endurance is reached. Men and 
material pass into and out of the caisson by means of the 
air-lock. The men within the caisson remove the material 
with which the lower edge of the caisson comes in contact 



1 82 COMPRESSED AIR. 

until a satisfactory foundation is reached, and the caisson 
is then built up full of substantial masonry and allowed to 
remain as a part of the permanent structure, which is con- 
tinued above it to any height desired. The caisson is now 
also frequently used in obtaining suitable foundations and 
supports for the tall and heavy office buildings erected in 
our large cities. 

Caissons^ Expelling Soft Material fro7n. — This is a use of 
compressed air entirely distinct from its primal function in 
the caisson of excluding the water so that the men may be 
able to work in it. When any soft material is found in the 
progress of the excavation, it is now customary to expel it 
through a pipe carried up through the top or side of the 
caisson, the pressure of the air within supplying all the power 
required. The pipe is provided with a quick-closing valve, 
so that when the material has all run out the air may not 
escape. 

Caissons^ Operating Air-lock JDoors, see Air-lock Doors. 

Cars^ Propelliiig^ on Street Railways. — Compressed air is 
not yet extensively used for this purpose in the United 
States, but is permanently established and successful in 
Paris and elsewhere in Europe. Experimental cars in the 
United States show excellent results, and the general adop- 
tion of the system in the near future is more than probable. 
In cost of plant, in facility of introduction, in economy of 
operation, and in the entire absence of objectionable feat- 
ures the compressed-air system surpasses all others. The 
principal delay as to its extensive introduction is in deter- 
mining the ultimately best of many details of construction 
and operation. 

Cars^ Dumping. — Cars dumped by compressed air are 
used in handling earthwork in railroad construction and 



A LIST OF THE VARIOUS APPLICATIONS, 1 83 

similar service, also for coal, ore, limestone, etc. An air- 
cylinder and piston under the car dumps the load upon 
either side as may be desired. An entire working train 
may be dumped at once, or a man may dump each car 
separately. 

Cars^ Loading. — One of the functions of the air-hoist or 
of the pneumatic crane. 

Cars, Unloading. — Unloading by lifting the load from 
the car by the air-hoist instead of by dumping. Oil-tank 
cars are discharged to a higher level by air pressure ad- 
mitted to the tank. 

Car Roofs, Sanding, see Sanding Car Roofs. 

Cars, Cleaning, — This system is now generally used at 
railroad termini. A supply of compressed air is main- 
tained, and a hose is led into the car or coach to be cleaned, 
with a nozzle for discharging the air and a cock for regu- 
lating or shutting it off. The jet of air is successively 
passed over the various parts of the interior of the car and 
the dust and other loose material is driven off at once. 

Car Seats, Cleaning. — The seats and cushions, rugs, etc. 
are removed from the car and, supported upon wooden 
horses, are thoroughly and quickly cleaned by the air jet. 

Car Sills, Fulling Down, see Pulling Down Jacks. 

Car Wheels and Axles, Removing, see Removing Car 
Wheels and Axles. 

Carriages, Gun, see Gun-carriages. 

Carpets, Cleaning. — The patented arrangement of the 
writer consists of a grated or perforated level floor or an 
inclined or vertical surface upon or against which the 
carpet to be cleaned is spread. The carpet is then tra- 
versed by an air-delivery pipe upon wheels and with handles 
like a lawn-mower. A hose conveys the air to the delivery- 



184 COMPRESSED AIR, 

pipe and it emerges in a series of fine jets close to tne sur- 
face of the carpet, rapidly expelling the dust and dirt. An 
exhaust fan draws the dust away whether liberated above 
or below the carpet. 

Castings^ Chippiiig, — One of the adaptations of the pneu- 
matic tool, which see. 

Caulking, — Now generally done by compressed air, espe- 
cially in boiler- and tank-work and upon the seams and 
joints of steel ships. This is another of the uses of the 
pneumatic tool. 

Cash-carriers, — Generally used in the large retail stores. 

Canal Locks or Lifts. — An important invention, lifting 
vessels to any height by a single lift, one air-lift taking the 
place of several of the old style of locks. As the lift is 
balanced, but little power is required to operate and little 
water is lost. 

Channelling-machines, — A modification of or more elabo- 
rate application of the rock drill, applied either to getting 
out stone of required shape and dimensions from its native 
bed, or cutting smooth channels in the solid rock, as at the 
Chicago Drainage Canal. 

Chemical Works. — In chemical works a supply of com- 
pressed air is constantly maintained and employed for 
various uses, such as the pneumatic pump or ejector, the 
air-lift and aerating processes. 

Cleaning, — Compressed air is employed in cleaning vari- 
ous things, such as flues, carpets, castings, by widely dif- 
ferent apparatus and processes. 

Chipping, — Another of the applications of the pneumatic 
tool, especially used in boiler-work, bridge- and ship-work, 
structural ironwork, foundries, etc. 

Clipping Horses. 



A LIST OF THE VARIOUS APPLICATIONS. 1 85 

Clocks, Operating. — In extensive use in Paris. Almost 
the only service rendered by compressed air that could be 
done as well or better by electricity. 

Coal Drills, Operating. — These are revolving drills or 
augers boring holes very rapidly for light charges of ex- 
plosives. 

Coal-mining Machines. — The cutting tool of the ma- 
chine reciprocates like a rock drill. It is mounted upon 
wheels and cuts under the seam of coal to a horizontal 
depth of five or six feet, when the coal may be broken 
down and removed. 

Coal or Culm Conveyors. 

Colors, Spraying. — Used in silk factories for spraying 
colors upon silk or satin ribbons. A recently perfected 
process sprays colors upon pottery, sometimes in liquid 
form and sometimes as a powder. Varied and novel effects 
are produced by applying several colors simultaneously by 
separate jets, or color and glazing may be mixed and 
sprayed together. 

Conductor's Train Signal. — Extensively used upon the 
best railroads. 

Cooling, — This is one of the general and widely appli- 
cable uses of compressed air. The fall of temperature in 
compressed air upon release is used for cooling drinking- 
water, for cooling houses or apartments, theatres, for gen- 
eral refrigeration, ice factories, and cold-storage warehouses. 

Copying-presses. 

Couplings, Applying, to Hose. 

Cranes, — Swinging, jib, or travelling cranes. 

Crossings, Gates at, see Gates at Railroad Crossings. 

Cupolas, Raising Stock to. 

Cutting off Stay-bolts. — This is done by a special style of 



1 86 COMPRESSED AIR. 

portable shears, requiring no skilled labor, and does not 
loosen the stay-bolt. 

Cuts and Qua} ries, Driving Machinery in. 

Dampening Lamidry-work. — The spraying-jet takes the 
place of the Chinaman's mouth, said to be employed for the 
same purpose. 

Dense-air Refrigerating Process. — Used upon warships 
and elsewhere. The same air is used over and over, com- 
pressed to say 15 atmospheres and expanded to say 5 atmos- 
pheres, and the same refrigerative effect is accomplished 
by less power and in smaller compass than when lower 
pressures are employed. 

Direct-acting Hoist, — Quicker and simpler than any other. 

Disappearing Gun-carriage. 

Disposal of Seivage, — The Shone ejector automatically 
raises the sewage to give it head to flow where the requisite 
grade cannot be maintained in the sewer. 

Distributing Sand on Locomotives. — Advantage is taken of 
the air-supply for the brakes, and the tracks are sanded, 
giving better adhesion and with less waste of sand. 

Diving-hell. 

Diving- arinor. 

Doors ^ Furnace, Raising and Loivering, 

Doors y Air- lock Doors, Operating, 

Doors, Opening, in Offices and Residences, — This is a sug- 
gested rather than an accomplished use of compressed air, 
but perfectly feasible where the air-supply exists. As we 
now automatically close our doors, so may we open them in 
welcome when any one approaches. 

Drainage Systems. — Compressed air is variously employed 
in such service the conditions determining the arrangement. 

Dredging. 



A LIST OF THE VARIOUS APPLICATIONS, 1 8/ 

Drills. — Revolving drills of various kinds, portable drills, 
metal drills, coal drills, diamond drills for prospecting. 

Drills^ Rock. — Reciprocating or percussion drills. The 
use of compressed air employing more of it than any other. 

Drinking' water ^ Cooling. 

Drinking-water^ Aerating. 

Driving Stay-bolt Tops. — One of the special uses of com- 
pressed air in the boiler-shop, simple, but saving much time 
and labor. 

Driving Machinery in Shops, — Two or three shops use 
compressed air for driving all their tools, dispensing with all 
shafting except a light line for a group of small tools. 

Driving Pumps, — An undeveloped use of compressed air 
of great importance and promise, see Chapter XVI. 

Driving Motors or Air-engines, 

Drop Pits. — This is the technical name for an arrange- 
ment in use in railroad repair shops. The car is run over 
the pit and an air-jack or hoist lowers wheels and axles to 
be removed or hoists new ones in place. 

Droppers for Cattle. — This name is given to one series of 
air-hoists used in the Armour packing-house and similar 
establishments. The bullock, suspended by the heels, after 
bleeding and decapitation is conveyed by a continuously 
travelling overhead railway to a hook on one of the drop- 
pers, the hook being held up by the pressure of the air with 
sufficient force to sustain the weight. Upon releasing the 
air the bullock is dropped upon the floor for skinning, dis- 
embowelling and such interesting operations. 

Drop Weight for Breaking Castings^ Lifting. — A popular 
use of compressed air in the yards of foundries that are 
fully equipped with it. A single hoisting cylinder is used 



1 88 COMPRESSED AIR, 

with multiplying sheaves so that the hoist of the weight is 
usually six or eight times the travel of the piston. 

Dry Dock, — The compressed-air dry dock may have all 
the advantages of the independent floating dock at less first 
cost and less cost of operation and maintenance. 

Dtwiping Cars, 

Dynamite Gun. — The only safe way yet devised for 
throwing the high explosives. Decided to be of value for 
coast defence. A number of these guns now under con- 
struction. 

Eggs^ Beating, 

Elevators. — Compressed air adapts itself readily to all the 
various demands of elevator service, and is used for passen- 
gers or freight, by direct or multiple lift of a single piston, 
or with an air-motor with gearing and drums. 

Elevators^ Coal and Culm, 

Elevators^ Indicators on, — Indicators operated by com- 
pressed air to signal or to inform the passenger and the 
operator. 

Ejector. — Used for automatically transferring sewage or 
other liquids. The air pressure being maintained, the 
chamber is alternately filled by the flow of the liquid, and 
emptied by its ejection or expulsion to a higher level. 

Engines., Fire., see Fire-engines. 

Engine Works., Driving, 

Expelling Soft Material from Caissons, 

Extinguishers^ Automatic Fire, 

Factories^ General Use in, — A great and rapidly increas- 
ing number of factories are equipped with compressed air, 
first of all for direct hoisting, and subsequently for various 
other purposes. 

Finish., Satin^ on Metal-work^ see Satin Finish. 



A LIST OF THE VARIOUS APPLICATIONS. 1 89 

Filtering Water. 

Fire-engines, — A suggested use of compressed air, per- 
fectly feasible wherever a general supply of compressed air 
is distributed. 

Fire-extinguisher. 

Fires ^ Blacksmiths\ Blowing. 

Fires, Kindling. — The aerated oil jet is used in many 
round-houses for starting the fires in the locomotives at 
one tenth of the cost of wood kindlings. 

Flues, Cleaning. 

Forcing Oil, — Transferring oil from tanks to barrels, or 
vice versa. 

Foundry, General Service in. 

Fountains, Cooling, see Chapter XIII. 

Furnace Doors, Raising and Lowering. 

Gas, Aerating. 

Gates at Crossings, Operating. — In connection with the 
compressed-air switch and signal service. 

Gear Steering, on Ships. 

General Hoisting Service. — Some establishments employ- 
ing more than a hundred air-hoists. 

Glass Factories. 

Glass-blowing. 

Grain Elevators. 

Granite-carving. 

Granite-cutting. — One of the uses of the pneumatic tool. 

Grates, Shaking. 

Gun carriage. Disappearing. 

Gun, Pneumatic. — Valuable for coast defence, throwing 
high explosives. 

Guns, Sporting or Target. 

Hammer, Fneu7?iatic. 



I go COMPRESSED AIR. 

Hardie Car-motor, 

Hoisting Cattle^ Beef. — Air-hoists used exclusively in the 
largest packing-houses. 

Hoist, Direct-acting Vertical-cylinder. 

Hoist, Geared, 

Horses, Clipping. 

Hose couplings, Applying. — The machine for this purpose 
is said to have paid for itself in one day's application of it. 

Hydraulic Cranes. — Air is used to give pressure to the 
water, while the water actually does the hoisting in some 
lines of service where the elasticity of the air would be 
objectionable. 

Hydraulic Pressure Relief. — In wood-pulp machines in 
paper-mills a hydraulic feed is employed which is some- 
times too positive, and a chamber of compressed air is pro- 
vided to relieve it and prevent breakage. 

Ice-making. 

Indicators on Elevators. 

Iron, Drills for. 

Iron Furnaces, Tapping. 

Iron Bridge Work. 

Ironwork, Structural. 

Jacks, Portable. 

Jacks, " Pulling Down.^^ 

Kiiidling Fires in Locomotives. 

Lamps. — Aerated oil lamps for street work, railroad op- 
erations, etc. 

Lard- refining. 

La undry- work. Dampening. 

Lifting Drop Weight in Foundry Yards. 

Locks, Canal. 

Lock Doors in Caissons, Operating. 



A LIST OF THE VARIOUS APPLICATIONS, I9I 

Loadijig Cars, 

Locoinotives in Mines, Street RailwaySy etc. 

Locomotives, Kindling Fires in. 

Medical Preparations, Spraying. 

Mekarski System of Car Propulsion, 

Mining Coal, — Compressed air is variously used in coal 
mines, for " coal-cutters/' coal augers, rock drills, pumps, 
hoists, etc. 

Mixing Nitroglycerine, 

Moulding-machines. — In the foundry compressed air rams 
or presses the sand in the moulding-machine, lifts the 
mould, draws the pattern, etc. 

Nitroglycerine, Mixing, 

Operating Air- drills and Punches 

Opening Doors, 

Packages, Transmitting, 

Painting, 

Pile-driver, 

Pits, Drop, see Drop Pits. 

Physicians' Spraying Apparatus, 

Pneumatic Ejector, 

Pneumatic Press, 

Pneumatic Signal for Railway Trains. 

Pneumatic Tool, 

Pnewnatic Tubes for Transmission. 

Portable Drill, 

Portable Jack, 

Preserving Timber, the Wood Vulcanizing Process, which 
see. 

Press, Copying, 

Press, Straightening, 

Pottery, Spraying with Colors^ 



192 . COMPRESSED AIR. 

Process^ Bessemer^ see Aerating Molten Metal. 

Pulling Down Jacks^ see Jacks, Pulling Down. 

Pump, Air- Lift. 

Pump, Auto7natic, 

Pu??ip, Beer. 

Punips, Operating. 

Pumping Acids. 

Punch, Portable. 

Punchiiig in Boiler-shops, etc. 

Quarries, General Work in. 

Railways, Street. 

Railroad Shops, Various Uses in. 

Railroad Shops and Sheds, Whitewashing. 

Raising Stock to Cupolas in Foundries. 

Raising Ships. — Air-tight bags are attached all around a 
sunken ship, or placed by divers in the hold, then inflated 
by compressed air, and, acting like balloons in the air, 
when their combined displacement is sufficient the ship 
rises. 

Refining Lard. 

Refining Asphalt, 

Refrigerating, 

Removing Mandrels. 

Rejnoving Scale from Steel Plates — another of the uses 
of the pneumatic tool. 

Ribbons, Spraying with Colors. 

Ringing Bells 07i Locomotives, 

Riveting. 

Rock Drills, Operating. 

Rock Tunnels, Driving, All Operations in. 

Sand-blast, 



A LIST OF THE VARIOUS APPLICATIONS, I93 

Sanding Tracks, — Giving better distribution, better adhe- 
sion, and wasting less sand than where delivered by gravity. 

Sa7iding Car Roofs, — A process used in the car-building 
or repair shops in connection with the painting of the roofs 
of freight cars. The sand is delivered by the air with force, 
so that it embeds itself in the paint, forming a protection 
for the surface. What is not held by the paint is removed 
by the same blast of air that delivers the sand. 

Satin Finish on Metals, — Used on plated work for rail- 
road cars. 

Scale y Removing^ from Steel Plates. 

Seats ^ Cleaniftg. 

Sewage Disposal. 

Sheathing File-driver, 

Sheep-shearing. 

Ships, Raising. 

Ships, Steering. 

Ships, General Service on. 

Shops, Driving Machine Tools in. 

Signal, Block. 

Signal, Conductor's, 

Silk Manufacture. 

Silk Ribbons^ Spraying. 

Skates. 

Soft-ground Tunnels. 

Spraying Laundry-work. 

Spraying Colors on Silk Ribbons, 

Spraying Colors on Fottery. 

Stay-bolts, Cutting off. 

Stay-bolt Taps, Driving, 

Steering-gear on Ships. 

Stone-cutting, 



194 COMPRESSED AIR. 

Storage^ Cold. 

Steel Plates^ Removing Scale from. 

Street Railways, 

Structural Ironwork. 

Switches and Signals on Railroads. 

Syrups^ Agitating. 

Taps, Driving, in Boiler-shops. 

Tapping Iron Furnaces. 

Testing Brakes, 

Timber, Preserving, 

Tires for Vehicles, 

Tool, Pneumatic, 

Torpedo Service, 

Tracks, Sanding. 

Train Signal. 

Transferring Oil or Acids. 

Transmitting Packages. 

Transmitting Power from Waterfalls. 

Travelling-crane, 

Trucks, Dumping. 

Tunnels, Soft-ground. 

Tunnels in Rock. 

Turrets, Operating on Warships, 

Unloading Cars, — This is done either by hoisting, by 
dumping, or in tank cars by pressure upon the service of 
the liquid. 

Ventilating, 

Vertical Direct Hoist. 

Vehicle Wheel-tires, 

Warfare, General use in. 

Water-balloon, see Raising Ships, 

Water ^ Raising, 



A LIST OF THE VAklOVS APPLICATIONS, tQS 

Water ^ Aerating, 

Water ^ Filtering. 

Wheels and Axles^ Hoisting or Removing, 

Whitewashing, 

Working Turrets. 

Worlds Fair Fai?tting, 

Wood Vulcanizing. 

Works, Chemical, Ge^ieral use in. 



INDEX 



Absolute temperature, ii. 

Absolutely isothermal or adiabatic compression impossible, 22. 

Accommodating attitude of air, 138. 

Action of air in passages of two-stage compressor, 75. 

Actual curve in expansion always above theoretical adiabatic, 102. 

Actual volume of air the basis in transmission computations, no. 

Additional lines on indicator-diagram, 43. 

Adiabatic and isothermal curves not required for computations, 51. 

Adiabatic compression, 15. 

Adiabatic curves to draw on diagram, 49. 

Advances in steam economy, 38. 

Air-brake pump, 128. 

Air-compression line simpler than the steam- expansion line, 49. 

Air-compressor as an air- meter, 54. 

Air-compressor is its own dynamometer, 39. 

Air-compressor diagram, 129. 

Air a political factor, 140. 

Air always contains moisture, 148. 

Air hoisting, 137. 

Air for operating pumps, 138. 

Air never freezes. 147. 

Air quickly heated or cooled, 95. 

Air readily receives or imparts heat, 63, 

Air the natural power-transmitter for mines, 175. 

Air used without cooling or draining, 150. 

Alternate resistance in single-acting two-stage tandem compressor, 83. 

Applications of air-compression diagram, 100. 

Back pressure in two-stage compression, 77. 

Bad luck of compressed air, 138. 

197 



198 INDEX, 

Bad practices of pipe-fitters, 114. 

Bad record of air for pumping and its causes, 169. 

Beginning of economical compression, 53. 

Best air-compressor practice, 31. 

Boiling-point variable, 11. 

Capacity of air for water, 149. 

Capacity of compressor as reduced by clearance, 60. 

Catalogues as diffusers of misinformation, 3 

Caution as to use of compression table, 23. 

Cold as possible air for compression, 54. 

Cold-water fountains, 144. 

Common working pressure for air, 70. 

Complicated operation of compression, 75. 

Compound compression, 25. 

Compressed-air diagram, explanation of, 16. 

Compressed air gives less power than equal volume of steam, 98. 

Compressed-air problem (the), 27. 

Compressed-air literature, 3. 

Compressed-air transmission, no. 

Compressed air versus electricity, 135. 

Compressed air widely used without freezing up, 148. 

Compressing cylinder always the first, 72. 

Compression completed in first cylinder of two-stage compressor. 72. 

Compression in a single cylinder, 61. 

Compression-line of air and steam expansion-line, 49. 

Compressors for continuous service, 134. 

Compressors in general use, 61. 

Computing M. E. R., 44. 

Computing I. H. P., 46. 

Computing power cost of compression, 90. 

Computing power required for compression, 23. 

Condition of interior of pipes, 113. 

Conditions of highest economy in compression, 134. 

Considerations of economy inapplicable, 4. 

Constant readiness of air, 137. 

Constant work under best conditions, 133. 

Continued transmission in winter, 154. 

Cooling air for caisson work, 156. 

Cooling drinking-water, 144. 

Cooling by injection, 67. 

Cobling by water-jacket, 64. 



INDEX. 199 

Cooling of air at release, 33. 

Corliss compressor, 133. 

Corliss feature for selling rather than for operating, 133. 

Cost of air-volume when produced by reheating, 158. 

Cost of compression only one part of question of economy, 90. 

Definitions and general information, 9. 

Devices for equalizing pressure to resistance, 130. 

Diagram from first cylinder does not vary with ultimate pressure, 72. 

Diagram for one volume of steam and air expanded, 99. 

Diagram for drawing adiabatic curve, 49. 

Diagram for drawing isothermal curve, 48. 

Diagram of steam and air expanded to one atmosphere, loi. 

Diagram of theoretical air-compression, 27. 

Diagram of practical air-compression, 29. 

Diagram of good comparison, 68. 

Diagram of volumes after reheating, 164. 

Diagram of pressures after reheating, 165. 

Diagram of volumetric relations of air and water, 142. 

Diagram showing no cooling of air in early part of stroke, 65. 

Diagrams from air-brake pump, 129. 

Diagrams from novel air-compressor, 132. 

Diagrams in compressor catalogues, 127. 

Diagrams of two-stage compression in single-acting cylinders, 71. 

Diagrams combined for double-acting cylinders, 84. 

Difference between diagrams from air and steam cylinders, 40. 

Difference between theoretical and actual temperatures, 26. 

Difference between theory and practice, 28. 

Differences in free-air volumes due to temperature, 55. 

Different effects of heat upon air and water, 141. 

Difficulty of learning the truth of air-compression practice, 126. 

Distinct operations in air-compression (two), 24. 

Distributing pipes, iii. 

Distribution of air in relation to intercooler, 85. 

Drawing the adiabatic curve, 49. 

Drawing the isothermal curve, 48. 

Drinking-fountains with warm water, 144. 

Dry air from injection compressors, 152. 

Dry-goods box and tumbler, 142. 

Economical compression, 53. 

Effect of heat on compressed air, 14. 

Effect of intercooler, 85. 



200 INDEX, 

Effect realized in mining-pumps, 36. 

Efficiencies in use of air, 33. 

Electric brake, 137. 

Electricity generally inapplicable for the work that compressed air 

does, 136. 
Electricity on railroads, 139. 
Entrained water in air-meters, 151. 
Erroneous ideas as to losses in transmission, iii. 
Example of pumping by air, 169. 
Examples of use of formula for flow of air, 11 7-1 19. 
Explanation of compressed-air diagram, 16. 
Explanation of general compression table, 20. 
Explanation of practical compression diagram, 30. 
Expansion of air by heat cheaper than steam production, 157. 
Factors in transmission computations, 112. 
Fahrenheit scale, II. 

False position accepted by compressed air. 
Five C's (the), 133. 
Fly-wheels on compressors, 130. 
Formula for friction of air in pipes, 115. 
Fountain cooled by compressed air, 144. 
Four sources of loss in air-compression, 93. 
Free air, definition, 10. 

Free air the raw material of compression, 54. 
Freezing up most frequent with low pressures, 155, 
Freezing up of air-motors, 34. 
Friction of engine, 39, 
Friction in straight-line compressors, 94. 
Gas-engine, 135. 

General Compressed Air Company, Where? 2. 
General compressed-air service, 145. 
Getting the air not only at but into the cylinder, 56. 
Giving a dog a bad name, 2. 
Governing the compressor, 134. 
Graphical study of two-stage compression, 81. 
Great changes of temperature with small transfers of heat, 143. 
Growing demand for compressed air, 6. 
Heat not evenly distributed in cylinder paits, 66. 
Heat mostly abstracted in latter part of stroke, 64. 
Heating and cooling of compressor parts, 63. 
Heating ceases when compression ceases, 65. 



INDEX. 201 

Heating effect of compression, 15. 

High-pressure air and dry air, 155. 

High pressures and pressure-reducers, 156. 

How not to do it, 35. 

Importance of the unimportant, 114. 

Indicator-diagram must not be taken too early, 41. 

Indicator does not tell weight of air compressed, 57, 

Indicator on the air-compressor (the), 38. 

Indicator peculiarly applied to air-compressor, 39. 

Intercooler, 86. 

Isothermal compression, 15. 

Isothermal curve, to draw, 48. 

Keeping the air cool means actual cooling, 62. 

Large compressors for continuous service, 134. 

Less than nothing to do, 82. 

Limits to reheating, 161. 

List of applications of air, 176. 

Little competition between air and electricity, 135. 

Little heat for reheating air, 157. 

Little power lost by clearance, 60. 

Little storage of air possible, 135. 

Loss, the word misleading, 36. 

Loss by elbows, 114. 

Loss by friction in two-stage compression, 80. 

Loss in compression not necessarily final, 31. 

Loss in transmission, 32. 

Loss in transmission not due to friction, 160. 

Loss of pressure compensated for by increase of volume, 32. 

Losses by hot free air, 57. 

Losses less in air-transmission than with any other transmitter, ill« 

Low friction in Corliss compressors, 94. 

M. E. R. for compression lower than for delivery, 73. 

M. E. R. different in single and in two-stage compression, 74. 

M. E. R. for compression only, 24. 

M. E. R. for whole stroke, 22. 

M. E. R. single and two-stage compound, 76. 

Measuring clearance, 44. 

Measuring total volume compressed, 44, 47. 

Mechanical versus commercial economy, i. 

More air-pressure behind piston than in front, 82. 

Much air to cool a little water, 145. 



202 INDEX. 

Novel arrangement for equalizing pressures, 130. 

Obstructions in pipes, 114. 

Oil-engine, 136. 

Operating pumps, 138. 

Operation of intercooler, 86. 

Paradox in use of air, 102. 

Peculiar position of compressed air, 2. 

Pipe conveying air to compressor, 55. 

Pohle air lift-pump, 143. 

Postal transmission, 5. 

Power cost of air, 90. 

Power required for hoisting and pumping, 167. 

Power value of air, 98. 

Practical man has no use for small figures, 55. 

Pumping a field for comparison with electricity, 174, 

Pumping should show high eflficiency, 168. 

Rapid cooling of air in pipes, 143. 

Ratio of cylinders, 71. 

Reading the compression-diagram, 38. 

Receiver near compressor does not dry the air, 151. 

Receiver needed after air is cooled, 152. 

Reheating, 34, 157. 

Reheating generally impracticable, 163. 

Reheating in Paris, 163. 

Reheating is doing work over again, 159. 

Relation of volume to temperature, 11. 

Relative work of low- and high-pressure cylindeKT '^ 

Ready-made pumps, 172. 

Rock-drill and air-compressor (the), 167. 

Saving by reheating, 34. 

Simultaneous heating and cooling, 63 

Single-acting tandem two-stage compressors, 77, 

Single cylinders mostly used, 70. 

Size of compression-cylinders, 69. 

Small compressors as missionaries, 8. 

Specific heat of air and of water, 14. 

Starting business under favorab e conditions, 62. 

Steam-pressures guarantee temperatures, 57. 

Steam-pumps, 169, 171. 

Summary of relations, 12. 

Table of weights and volumes of dry air, 13. 



INDEX, 2QI 

Table of volumes, mean pressures, etc., 19. 

Table of final temperatures, 37. 

Table of absolute pressures, boiling-points, etc., 52. 

Table of power required to compress air, 92. 

Table of mean effective and terminal pressures, 103-108. 

Table of volumes of air flowing in pipes, 109. 

Table of relative volumes of air at different pressures, 119. 

Table of head required to overcome friction in pipes, 121-125. 

Temperature of air in cylinder not ascertainable, 58. 

Thermal relations of air and water, 141. 

Theoretical compression, 28. 

Transmission formulas unsatisfactory, 112. 

Triumph of the steam-engineer in electrical developments, 139. 

Tumbler and the dry-goods box, 142. 

Two-stage compression, 31, 70. 

Two-stage compression in single-acting cylinders, 71. 

Two things combine to cause freezing up, 148. 

Unique among power-transmitters, 5. 

Unique opportunity of electricity, 170. 

Unit of heat, 14. 

Ultimate economy in reheating, 162. 

Up-to-date compressor (the), 126. 

Use of air for old steam pumps, 35. 

Use of air in railroad shops, 7. 

Use of table of power cost, 93. 

Various efficiencies in use of air, 33. 

Various ratios of the four losses in compression, 97. 

Vindication of air-brake pump, 128. 

Warm air in blowing cylinders, 59. 

Water in compression-cylinders and lubrication, 67. 

Water-jacket, 64. 

Water will not wet what is wet, 153. 

Wastefulness of air-brake pump, 4. 

Wet compressor furnishes driest air, 153. 

Whitewashing apparatus, 6. 

Without loss or gain of heat, 15. 

Worst air-compressor in existence, 128. 



